Plug Power Inc. 11,000,000 Shares Common Stock ($.01 par value) Underwriting Agreement

Exhibit 1.1

[EXECUTION VERSION]

Plug Power Inc.

11,000,000 Shares

Common Stock

($.01 par value)

Underwriting Agreement

New York, New York

August 4, 2005

Citigroup Global Markets Inc.

As Representatives of the Several Underwriters

c/o Citigroup Global Markets Inc.

000 Xxxxxxxxx Xxxxxx

Xxx Xxxx, Xxx Xxxx 00000

Ladies and Gentlemen:

Plug Power Inc., a corporation organized under the laws of Delaware (the “Company”), proposes to sell to the several underwriters named in Schedule II hereto (the “Underwriters”), for whom you (the “Representatives”) are acting as representatives, the number of shares of Common Stock, $.01 par value (“Common Stock”), of the Company set forth in Schedule I hereto (said shares to be issued and sold by the Company being hereinafter called the “Underwritten Securities”). The Company also proposes to grant to the Underwriters an option to purchase up to the number of additional shares of Common Stock set forth in Schedule II hereto to cover over-allotments (the “Option Securities”; the Option Securities, together with the Underwritten Securities, being hereinafter called the “Securities”). To the extent there are no additional Underwriters listed on Schedule II other than you, the term Representatives as used herein shall mean you, as Underwriters, and the terms Representatives and Underwriters shall mean either the singular or plural as the context requires. Any reference herein to the Registration Statement, the Basic Prospectus, any Preliminary Final Prospectus or the Final Prospectus shall be deemed to refer to and include the documents incorporated by reference therein pursuant to Item 12 of Form S-3 which were filed under the Exchange Act on or before the Effective Date of the Registration Statement or the issue date of the Basic Prospectus, any Preliminary Final Prospectus or the Final Prospectus, as the case may be; and any reference herein to the terms “amend”, “amendment” or “supplement” with respect to the Registration Statement, the Basic Prospectus, any Preliminary Final Prospectus or the Final Prospectus shall be deemed to refer to and include the filing of any document under the Exchange Act after the Effective Date of the Registration Statement or the issue date of the Basic Prospectus, any Preliminary Final Prospectus or the Final Prospectus, as the case may be, deemed to be incorporated therein by reference. Certain terms used herein are defined in Section 18 hereof.

1. Representations and Warranties. The Company represents and warrants to, and agrees with, each Underwriter as set forth below in this Section 1.

(a) The Company meets the requirements for use of Form S-3 under the Act and has prepared and filed with the Commission a registration statement (file number 333-117358) on Form S-3, including a related basic prospectus, for registration under the Act of the offering and sale of the Securities. The Company may have filed one or more amendments thereto, including a Preliminary Final Prospectus, each of which has previously been furnished to you. The Company will next file with the Commission one of the following: (1) after the Effective Date of such registration statement, a final prospectus supplement relating to the Securities in accordance with Rules 430A and 424(b), (2) prior to the Effective Date of such registration statement, an amendment to such registration statement (including the form of final prospectus supplement) or (3) a final prospectus in accordance with Rules 415 and 424(b). In the case of clause (1), the Company has included in such registration statement, as amended at the Effective Date, all information (other than Rule 430A Information) required by the Act and the rules thereunder to be included in such registration statement and the Final Prospectus. As filed, such final prospectus supplement or such amendment and form of final prospectus supplement shall contain all Rule 430A Information, together with all other such required information, and, except to the extent the Representatives shall agree in writing to a modification, shall be in all substantive respects in the form furnished to you prior to the Execution Time or, to the extent not completed at the Execution Time, shall contain only such specific additional information and other changes (beyond that contained in the Basic Prospectus and any Preliminary Final Prospectus) as the Company has advised you, prior to the Execution Time, will be included or made therein. The Registration Statement, at the Execution Time, meets the requirements set forth in Rule 415(a)(1)(x).

(b) On the Effective Date, the Registration Statement did or will, and when the Final Prospectus is first filed (if required) in accordance with Rule 424(b) and on the Closing Date (as defined herein) and on any date on which Option Securities are purchased, if such date is not the Closing Date (a “Settlement Date”), the Final Prospectus (and any supplement thereto) will, comply in all material respects with the applicable requirements of the Act and the Exchange Act and the respective rules thereunder; on the Effective Date and at the Execution Time, the Registration Statement did not or will not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary in order to make the statements therein not misleading; and, on the Effective Date, the Final Prospectus, if not filed pursuant to Rule 424(b), will not, and on the date of any filing pursuant to Rule 424(b) and on the Closing Date and any Settlement Date, the Final Prospectus (together with any supplement thereto) will not, include any untrue statement of a material fact or omit to state a material fact necessary in order to make the statements therein, in the light of the circumstances under which they were made, not misleading; provided, however, that the Company makes no representations or warranties as to the information contained in or omitted from the Registration Statement or the Final Prospectus (or any supplement thereto) in reliance upon and in conformity with information furnished in writing to the Company by or on behalf of any Underwriter through the Representatives specifically for

inclusion in the Registration Statement or the Final Prospectus (or any supplement thereto).

(c) Each of the Company and its Subsidiaries has been duly incorporated and is validly existing as a corporation in good standing under the laws of the jurisdiction in which it is chartered or organized with full corporate power and authority to own or lease, as the case may be, and to operate its properties and conduct its business as described in the Final Prospectus; and is duly qualified to do business as a foreign corporation and is in good standing under the laws of each jurisdiction which requires such qualification except where the failure to be so qualified or in good standing could not reasonably be expected to have a material adverse effect on the condition (financial or otherwise), prospects, earnings, business or properties of the Company and its Subsidiaries, taken as a whole, whether or not arising from transactions in the ordinary course of business, except for matters set forth in or contemplated in the Final Prospectus (exclusive of any supplement thereto) (a “Material Adverse Effect”).

(d) All the outstanding shares of capital stock of each Subsidiary of the Company have been duly and validly authorized and issued and are fully paid and nonassessable, and, except as otherwise set forth in the Final Prospectus, all outstanding shares of capital stock of the Subsidiaries are owned by the Company free and clear of any perfected security interest or any other security interests, claims, liens or encumbrances.

(e) The Company’s authorized equity capitalization is as set forth or included in the Final Prospectus; the capital stock of the Company conforms in all material respects to the description thereof contained in the Final Prospectus; the outstanding shares of Common Stock have been duly and validly authorized and issued and are fully paid and nonassessable; the Securities have been duly and validly authorized, and, when issued and delivered to and paid for by each Underwriter pursuant to this Agreement, will be fully paid and nonassessable; the Securities are duly listed, and admitted and authorized for trading, subject, in the case of the Securities to be issued and sold by the Company, to official notice of issuance, on the Nasdaq National Market (“Nasdaq”); the Common Stock of the Company is quoted on Nasdaq and the Company is not in violation of the listing requirements of Nasdaq. There is no stop order suspending the trading of the Company’s Common Stock on Nasdaq and, except as otherwise disclosed in the Registration Statement and the Final Prospectus, the Company is not aware of any state of fact or circumstance which could result in the Common Stock being delisted from Nasdaq; the certificates for the Securities are in valid and sufficient form; and the holders of outstanding shares of capital stock of the Company are not entitled to preemptive or other rights to subscribe for the Securities; and, except as set forth in the Final Prospectus, no options, warrants or other rights to purchase, agreements or other obligations to issue, or rights to convert any obligations into or exchange any securities for, shares of capital stock of or ownership interests in the Company are outstanding.

(f) There is no franchise, contract or other document of a character required to be described by the Act in the Registration Statement or Final Prospectus, or to be filed

as an exhibit thereto, which is not described or filed as required; and the statements in the Final Prospectus, insofar as such statements summarize legal matters, agreements, documents or proceedings discussed therein, are, in all material respects, accurate and fair summaries of such legal matters, agreements, documents or proceedings.

(g) This Agreement has been duly authorized, executed and delivered by the Company and constitutes a valid and binding obligation of the Company enforceable in accordance with its terms, except as enforceability may be limited by applicable bankruptcy, insolvency, reorganization, moratorium or similar laws affecting creditors’ and contracting parties’ rights generally and except as enforceability may be subject to general principles of equity.

(h) The Company is not and, after giving effect to the offering and sale of the Securities and the application of the proceeds thereof as described in the Final Prospectus, will not be an “investment company” as defined in the Investment Company Act of 1940, as amended.

(i) No consent, approval, authorization, filing with or order of any court or governmental agency or body is required in connection with the transactions contemplated herein, except such as have been obtained under the Act and such as may be required under the blue sky laws of any jurisdiction in connection with the purchase and distribution of the Securities by the Underwriters in the manner contemplated herein and in the Final Prospectus.

(j) Neither the issue and sale of the Securities nor the consummation of any other of the transactions herein contemplated nor the fulfillment of the terms hereof will conflict with, result in a breach or violation of, or imposition of any lien, charge or encumbrance upon any property or assets of the Company or any of its Subsidiaries pursuant to, (i) the charter or bylaws of the Company or any of its Subsidiaries, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which the Company or any of its Subsidiaries is a party or bound or to which its or their property is subject, or (iii) any statute, law, rule, regulation, judgment, order or decree applicable to the Company or any of its Subsidiaries of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority, including Nasdaq, having jurisdiction over the Company or any of its Subsidiaries or any of its or their properties, except, in the case of clauses (ii) and (iii), for any breaches, conflicts or violations which could not reasonably be expected to have a Material Adverse Effect.

(k) No holders of securities of the Company have rights to the registration of such securities under the Registration Statement (except any such rights which have been waived).

(l) The consolidated historical financial statements and schedules of the Company and its consolidated Subsidiaries incorporated by reference in the Final Prospectus and the Registration Statement present fairly in all material respects the financial condition, results of operations and cash flows of the Company as of the dates

and for the periods indicated, comply as to form with the applicable accounting requirements of the Act and have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods involved. The selected financial data in the Final Prospectus and Registration Statement fairly present the information included therein.

(m) No action, suit or proceeding by or before any court or governmental agency, authority or body or any arbitrator involving the Company or any of its Subsidiaries or its or their property is pending or, to the knowledge of the Company, threatened that (i) could reasonably be expected to have a material adverse effect on the performance of this Agreement or the consummation of any of the transactions contemplated hereby or (ii) could reasonably be expected to have a Material Adverse Effect.

(n) Neither the Company nor any Subsidiary is in violation or default of (i) any provision of its charter or bylaws, (ii) the terms of any indenture, contract, lease, mortgage, deed of trust, note agreement, loan agreement or other agreement, obligation, condition, covenant or instrument to which it is a party or bound or to which its property is subject or (iii) any statute, law, rule, regulation, judgment, order or decree of any court, regulatory body, administrative agency, governmental body, arbitrator or other authority having jurisdiction over the Company or such Subsidiary or any of its properties, as applicable, except, in the case of clause (ii) and (iii), for violations or defaults that could not reasonably be expected to have a Material Adverse Effect.

(o) KPMG LLP, who have certified certain financial statements of the Company and its consolidated Subsidiaries and delivered their report with respect to the audited consolidated financial statements and schedules incorporated by reference in the Final Prospectus, are an independent registered public accounting firm with respect to the Company within the meaning of the Act and the applicable published rules and regulations thereunder.

(p) There are no transfer taxes or other similar fees or charges under federal law or the laws of any state, or any political subdivision thereof, required to be paid in connection with the execution and delivery of this Agreement or the issuance by the Company or sale by the Company of the Securities.

(q) The Company has filed all foreign, federal, state and local tax returns that are required to be filed or has requested extensions thereof (except in any case in which the failure so to file would not have a Material Adverse Effect and has paid all taxes required to be paid by it and any other assessment, fine or penalty levied against it, to the extent that any of the foregoing is due and payable, except for any such assessment, fine or penalty that is currently being contested in good faith or as would not have a Material Adverse Effect.

(r) No labor problem or dispute with the employees of the Company or any of its Subsidiaries exists or is threatened or imminent, and the Company is not aware of any

existing or imminent labor disturbance by the employees of any of its or its Subsidiaries’ principal suppliers, contractors or customers that could have a Material Adverse Effect.

(s) The Company and each of its Subsidiaries are insured by insurers of recognized financial responsibility against such losses and risks and in such amounts as are prudent and customary in the businesses in which they are engaged; all policies of insurance insuring the Company or any of its Subsidiaries or their respective businesses, assets, employees, officers and directors are in full force and effect.

(t) The Company and its Subsidiaries are (i) in compliance with any and all applicable foreign, federal, state and local laws and regulations relating to the protection of human health and safety, the environment or hazardous or toxic substances or wastes, pollutants or contaminants (“Environmental Laws”), (ii) have received and are in compliance with all permits, licenses or other approvals required of them under applicable Environmental Laws to conduct their respective businesses and (iii) have not received notice of any actual or potential liability under any Environmental Law, except where such non-compliance with Environmental Laws, failure to receive required permits, licenses or other approvals, or liability would not, individually or in the aggregate, have a Material Adverse Effect. Except as set forth in the Final Prospectus, neither the Company nor any of the Subsidiaries has been named as a “potentially responsible party” under the Comprehensive Environmental Response, Compensation, and Liability Act of 1980, as amended.

(v) The Company, in its reasonable judgment, has concluded that any costs and liabilities associated with Environmental Laws (including, without limitation, any capital or operating expenditures required for clean-up, closure of properties or compliance with Environmental Laws, or any permit, license or approval, any related constraints on operating activities and any potential liabilities to third parties) would not, singly or in the aggregate, have a Material Adverse Effect.

(w) Neither the Company nor any of its Subsidiaries nor, to the knowledge of the Company, any director, officer, agent, employee or affiliate of the Company or any of its Subsidiaries is aware of or has taken any action, directly or indirectly, that would result in a violation by such persons of the FCPA, including, without limitation, making use of the mails or any means or instrumentality of interstate commerce corruptly in furtherance of an offer, payment, promise to pay or authorization of the payment of any money, or other property, gift, promise to give, or authorization of the giving of anything of value to any “foreign official” (as such term is defined in the FCPA) or any foreign political party or official thereof or any candidate for foreign political office, in contravention of the FCPA.

(x) The operations of the Company and its Subsidiaries are and have been conducted at all times in compliance, in all material respects, with applicable financial recordkeeping and reporting requirements of the Currency and Foreign Transactions Reporting Act of 1970, as amended, the money laundering statutes of all jurisdictions, the rules and regulations thereunder and any related or similar rules, regulations or

guidelines, issued, administered or enforced by any governmental agency (collectively, the “Money Laundering Laws”).

(y) Neither the Company nor any of its Subsidiaries nor, to the knowledge of the Company, any director, officer, agent, employee or affiliate of the Company or any of its Subsidiaries is currently subject to any U.S. sanctions administered by the Office of Foreign Assets Control of the U.S. Treasury Department (“OFAC”); and the Company will not directly or indirectly use the proceeds of the offering, or lend, contribute or otherwise make available such proceeds to any Subsidiary, joint venture partner or other person or entity, for the purpose of financing the activities of any person currently subject to any U.S. sanctions administered by OFAC.

(z) The Company and its Subsidiaries possess all licenses, certificates, permits and other authorizations issued by the appropriate federal, state or foreign regulatory authorities necessary to conduct their respective businesses, except for failures to possess any licenses, certificates, permits or other authorizations that would not reasonably be expected to have a Material Adverse Effect and neither the Company nor any such Subsidiary has received any notice of proceedings relating to the revocation or modification of any such certificate, authorization or permit which, singly or in the aggregate, if the subject of an unfavorable decision, ruling or finding, would have a Material Adverse Effect.

(aa) The Company and each of its Subsidiaries maintain a system of internal accounting controls sufficient to provide reasonable assurance that (i) transactions are executed in accordance with management’s general or specific authorizations; (ii) transactions are recorded as necessary to permit preparation of financial statements in conformity with generally accepted accounting principles and to maintain asset accountability; (iii) access to assets is permitted only in accordance with management’s general or specific authorization; and (iv) the recorded accountability for assets is compared with the existing assets at reasonable intervals and appropriate action is taken with respect to any differences. The Company maintains disclosure controls and procedures (as such term is defined in Rule 13a-15 under the Exchange Act) that are procedures (as such term is defined in Rule 13a-15 under the Exchange Act) that are designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the Exchange Act is recorded, processed, summarized and reported, within the time periods specified in the rules and forms of the Commission, including, without limitation, controls and procedures designed to ensure that information required to be disclosed by the Company in the reports that it files or submits under the Exchange Act is accumulated and communicated to the Company’s management, including its principal executive officer or officers and its principal financial officer or officers, as appropriate to allow timely decisions regarding required disclosure.

(bb) The Company has not taken, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, under

the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Securities.

(cc) The Company owns or has the right to use the inventions, patent applications, patents, trademarks (both registered and unregistered), trade names, copyrights and trade secrets used in the conduct of the Company’s business as currently conducted (collectively, the “Intellectual Property”); the Company has not received any written communication alleging that any Intellectual Property is invalid or unenforceable, that could reasonably be expected to have a Material Adverse Effect; and there is no pending or, to the Company’s knowledge, threatened action, suit, proceeding or claim by others that the Company’s use of the Intellectual Property infringes or otherwise violates any patent, trademark, copyright, trade secret or other proprietary right of others that could reasonably be expected to have a Material Adverse Effect.

(dd) There is and has been no material failure on the part of the Company or any of the Company’s directors or officers, in their capacities as such, to comply with any provision of the Sarbanes Oxley Act of 2002 and the rules and regulations promulgated in connection therewith, including Section 402 related to loans and Sections 302 and 906 related to certifications.

Any certificate signed by any officer of the Company and delivered to the Representatives or counsel for the Underwriters in connection with the offering of the Securities shall be deemed a representation and warranty by the Company, as to matters covered thereby, to each Underwriter.

2. Purchase and Sale. (a) Subject to the terms and conditions and in reliance upon the representations and warranties herein set forth, the Company agrees to sell to each Underwriter, and each Underwriter agrees, severally and not jointly, to purchase from the Company, at a purchase price of $6.25 per share, the amount of the Underwritten Securities set forth opposite such Underwriter’s name in Schedule II hereto.

(b) Subject to the terms and conditions and in reliance upon the representations and warranties herein set forth, the Company hereby grants an option to the several Underwriters to purchase, severally and not jointly, up to the number of Option Securities at the same purchase price per share as the Underwriters shall pay for the Underwritten Securities. Said option may be exercised only to cover over-allotments in the sale of the Underwritten Securities by the Underwriters. Said option may be exercised in whole or in part at any time on or before the 30th day after the date of the Final Prospectus upon written or telegraphic notice by the Representatives to the Company setting forth the number of shares of the Option Securities as to which the several Underwriters are exercising the option and the Settlement Date. The number of shares of the Option Securities to be purchased by each Underwriter shall be the same percentage of the total number of shares of the Option Securities to be purchased by the several Underwriters as such Underwriter is purchasing of the Underwritten Securities, subject to such adjustments as you in your absolute discretion shall make to eliminate any fractional shares.

3. Delivery and Payment. (a) Delivery of and payment for the Underwritten Securities and the Option Securities (if the option provided for in Section 2(b) hereof shall have

been exercised on or before the third Business Day prior to the Closing Date) shall be made on the date and at the time specified in Schedule I hereto or at such time on such later date not more than three Business Days after the foregoing date as the Representatives shall designate, which date and time may be postponed by agreement between the Representatives and the Company or as provided in Section 9 hereof (such date and time of delivery and payment for the Securities being herein called the “Closing Date”). Delivery of the Securities shall be made to the Representatives for the respective accounts of the several Underwriters against payment by the several Underwriters through the Representatives of the purchase price thereof to or upon the order of the Company by wire transfer payable in same-day funds to an account specified by the Company. Delivery of the Underwritten Securities and the Option Securities shall be made through the facilities of The Depository Trust Company unless the Representatives shall otherwise instruct.

(b) If the option provided for in Section 2(b) hereof is exercised after the third Business Day prior to the Closing Date, the Company will deliver the Option Securities (at the expense of the Company) to the Representatives, c/o Citigroup Global Markets Inc. at 000 Xxxxxxxxx Xxxxxx, Xxx Xxxx, Xxx Xxxx, xx the date specified by the Representatives (which shall be within three Business Days after exercise of said option) for the respective accounts of the several Underwriters, against payment by the several Underwriters through the Representatives of the purchase price thereof to or upon the order of the Company by wire transfer payable in same-day funds to an account specified by the Company. If settlement for the Option Securities occurs after the Closing Date, the Company will deliver to the Representatives on the Settlement Date for the Option Securities, and the obligation of the Underwriters to purchase the Option Securities shall be conditioned upon receipt of, supplemental opinions, certificates and letters confirming as of such date the opinions, certificates and letters delivered on the Closing Date pursuant to Section 6 hereof.

4. Offering by Underwriters. It is understood that the several Underwriters propose to offer the Securities for sale to the public as set forth in the Final Prospectus.

5. Agreements. The Company agrees with the several Underwriters that:

(a) The Company will use its best efforts to cause the Registration Statement, if not effective at the Execution Time, and any amendment thereof, to become effective. Prior to the termination of the offering of the Securities, the Company will not file any amendment of the Registration Statement or supplement (including the Final Prospectus or any Preliminary Final Prospectus) to the Basic Prospectus or any Rule 462(b) Registration Statement unless the Company has furnished you a copy for your review prior to filing and will not file any such proposed amendment or supplement to which you reasonably object. Subject to the foregoing sentence, if the Registration Statement has become or becomes effective pursuant to Rule 430A, or filing of the Final Prospectus is otherwise required under Rule 424(b), the Company will cause the Final Prospectus, properly completed, and any supplement thereto to be filed in a form approved by the Representatives with the Commission pursuant to the applicable paragraph of Rule 424(b) within the time period prescribed and will provide evidence satisfactory to the Representatives of such timely filing. The Company will

promptly advise the Representatives (1) when the Registration Statement, if not effective at the Execution Time, shall have become effective, (2) when the Final Prospectus, and any supplement thereto, shall have been filed (if required) with the Commission pursuant to Rule 424(b) or when any Rule 462(b) Registration Statement shall have been filed with the Commission, (3) when, prior to termination of the offering of the Securities, any amendment to the Registration Statement shall have been filed or become effective, (4) of any request by the Commission or its staff for any amendment of the Registration Statement, or any Rule 462(b) Registration Statement, or for any supplement to the Final Prospectus or for any additional information, (5) of the issuance by the Commission of any stop order suspending the effectiveness of the Registration Statement or the institution or threatening of any proceeding for that purpose and (6) of the receipt by the Company of any notification with respect to the suspension of the qualification of the Securities for sale in any jurisdiction or the institution or threatening of any proceeding for such purpose. The Company will use its reasonable best efforts to prevent the issuance of any such stop order or the suspension of any such qualification and, if issued, to obtain as soon as possible the withdrawal thereof.

(b) If, at any time when a prospectus relating to the Securities is required to be delivered under the Act, any event occurs as a result of which the Final Prospectus as then supplemented would include any untrue statement of a material fact or omit to state any material fact necessary to make the statements therein in the light of the circumstances under which they were made not misleading, or if it shall be necessary to amend the Registration Statement or supplement the Final Prospectus to comply with the Act or the Exchange Act or the respective rules thereunder, the Company promptly will (1) notify the Representatives of such event, (2) prepare and file with the Commission, subject to the second sentence of paragraph (a) of this Section 5, an amendment or supplement which will correct such statement or omission or effect such compliance and (3) supply any supplemented Final Prospectus to you in such quantities as you may reasonably request.

(c) As soon as practicable, the Company will make generally available to its security holders and to the Representatives an earnings statement or statements of the Company and its Subsidiaries which will satisfy the provisions of Section 11(a) of the Act and Rule 158 under the Act.

(d) The Company will furnish to the Representatives and counsel for the Underwriters, without charge, signed copies of the Registration Statement (including exhibits thereto) and to each other Underwriter a copy of the Registration Statement (without exhibits thereto) and, so long as delivery of a prospectus by an Underwriter or dealer may be required by the Act, as many copies of each Preliminary Final Prospectus and the Final Prospectus and any supplement thereto as the Representatives may reasonably request. The Company will pay the expenses of printing or other production of all documents relating to the offering.

(e) The Company will use its best efforts to arrange, if necessary, for the qualification of the Securities for sale under the laws of such jurisdictions as the Representatives may designate, will maintain such qualifications in effect so long as required for the distribution of the Securities and will pay any fee of the National Association of Securities Dealers, Inc., in connection with its review of the offering; provided, that in no event shall the Company be obligated to qualify to do business in any jurisdiction where it is not now so qualified or to take any action that would subject it to service of process in suits, other than those arising out of the offering or sale of the Securities, in any jurisdiction where it is not now so subject.

(f) The Company will not, without the prior written consent of Citigroup Global Markets Inc., offer, sell, contract to sell, pledge, or otherwise dispose of, (or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition due to cash settlement or otherwise) by the Company or any affiliate of the Company or any person in privity with the Company or any affiliate of the Company) directly or indirectly, including the filing (or participation in the filing) of a registration statement with the Commission in respect of, or establish or increase a put equivalent position or liquidate or decrease a call equivalent position within the meaning of Section 16 of the Exchange Act, any other shares of Common Stock or any securities convertible into, or exercisable, or exchangeable for, shares of Common Stock; or publicly announce an intention to effect any such transaction, until the Business Day set forth on Schedule I hereto, provided, however, that the Company may issue and sell Common Stock pursuant to any employee stock option plan, stock ownership plan or dividend reinvestment plan of the Company in effect at the Execution Time and the Company may issue Common Stock issuable upon the conversion of securities or the exercise of warrants outstanding at the Execution Time.

(g) The Company will not consent to the sale of more than an aggregate of 75,000 shares of Common Stock by its officers during the period beginning on the date hereof and ending on the Business Day set forth on Schedule I hereof.

(h) The Company will, during the period when a prospectus is required to be delivered, comply, in all material respects, with all applicable securities and other applicable laws, rules and regulations, including, without limitation, the Sarbanes Oxley Act, and to use its reasonable efforts to cause the Company’s directors and officers, in their capacities as such, during the period when a prospectus is required to be delivered, to comply, in all material respects, with such laws, rules and regulations, including, without limitation, the provisions of the Sarbanes Oxley Act.

(i) The Company will not take, directly or indirectly, any action designed to or that would constitute or that might reasonably be expected to cause or result in, under the Exchange Act or otherwise, stabilization or manipulation of the price of any security of the Company to facilitate the sale or resale of the Securities.

6. Conditions to the Obligations of the Underwriters. The obligations of the Underwriters to purchase the Underwritten Securities and the Option Securities, as the case may

be, shall be subject to the accuracy of the representations and warranties on the part of the Company contained herein as of the Execution Time, the Closing Date and any Settlement Date pursuant to Section 3 hereof, to the accuracy of the statements of the Company made in any certificates pursuant to the provisions hereof, to the performance by the Company of its obligations hereunder and to the following additional conditions:

(a) If the Registration Statement has not become effective prior to the Execution Time, unless the Representatives agree in writing to a later time, the Registration Statement will become effective not later than (i) 6:00 PM New York City time on the date of determination of the public offering price, if such determination occurred at or prior to 3:00 PM New York City time on such date or (ii) 9:30 AM on the Business Day following the day on which the public offering price was determined, if such determination occurred after 3:00 PM New York City time on such date; if filing of the Final Prospectus, or any supplement thereto, is required pursuant to Rule 424(b), the Final Prospectus, and any such supplement, will be filed in the manner and within the time period required by Rule 424(b); and no stop order suspending the effectiveness of the Registration Statement shall have been issued and no proceedings for that purpose shall have been instituted or threatened.

(b) (1) The Company shall have requested and caused Xxxxxxx Procter LLP, counsel for the Company, to have furnished to the Representatives their opinion, dated the Closing Date and addressed to the Representatives, to the effect set forth in Exhibit B and (2) The Company shall have requested and caused Xxxxxx X. Xxxxxx, Xx., counsel for the Company, to have furnished to the Representatives his opinion, dated the Closing Date and addressed to the Representatives, to the effect set forth in Exhibit C.

(c) The Representatives shall have received from King & Spalding LLP, counsel for the Underwriters, such opinion or opinions, dated the Closing Date and addressed to the Representatives, with respect to the issuance and sale of the Securities, the Registration Statement, the Final Prospectus (together with any supplement thereto) and other related matters as the Representatives may reasonably require, and the Company shall have furnished to such counsel such documents as they request for the purpose of enabling them to pass upon such matters.

(d) The Company shall have furnished to the Representatives a certificate of the Company, signed by the Chairman of the Board or the President and the principal financial or accounting officer of the Company, dated the Closing Date, to the effect that the signers of such certificate have reviewed the Registration Statement, the Final Prospectus, any supplements to the Final Prospectus and this Agreement and that:

(i) the representations and warranties of the Company in this Agreement are true and correct on and as of the Closing Date with the same effect as if made on the Closing Date and the Company has complied with all the agreements and satisfied all the conditions on its part to be performed or satisfied at or prior to the Closing Date;

(ii) no stop order suspending the effectiveness of the Registration Statement has been issued and no proceedings for that purpose have been instituted or, to the Company’s knowledge, threatened; and

(iii) since the date of the most recent financial statements included or incorporated by reference in the Final Prospectus (exclusive of any supplement thereto), there has been no material adverse effect on the condition (financial or otherwise), prospects, earnings, business or properties of the Company and its Subsidiaries, taken as a whole, whether or not arising from transactions in the ordinary course of business, except as for matters set forth in or contemplated in the Final Prospectus (exclusive of any supplements thereto).

(e) The Company shall have requested and caused KPMG LLP to have furnished to the Representatives, at the Execution Time and at the Closing Date, letters (which may refer to letters previously delivered to one or more of the Representatives), dated respectively as of the Execution Time and as of the Closing Date, in form and substance satisfactory to the Representatives, confirming that they are an independent registered public accounting firm within the meaning of the Act and the Exchange Act and the respective applicable rules and regulations adopted by the Commission thereunder and that they have performed a review of the unaudited interim financial information of the Company for the three- and six-month period ended June 30, 2005, and as at June 30, 2005 in accordance with Statement on Auditing Standards No. 100, and stating in effect, except as provided in Schedule I hereto, that:

(i) in their opinion the audited financial statements and financial statement schedules included or incorporated by reference in the Registration Statement and the Final Prospectus and reported on by them comply as to form in all material respects with the applicable accounting requirements of the Act and the Exchange Act and the related rules and regulations adopted by the Commission;

(ii) on the basis of a reading of the latest unaudited financial statements made available by the Company and its Subsidiaries; their limited review, in accordance with standards established under Statement on Auditing Standards No. 100, of the unaudited interim financial information for the three- and six-month period ended June 30, 2005, and as at June 30, 2005, incorporated by reference in the Registration Statement and the Final Prospectus; carrying out certain specified procedures (but not an examination in accordance with generally accepted auditing standards) which would not necessarily reveal matters of significance with respect to the comments set forth in such letter; a reading of the minutes of the meetings of the stockholders, directors and audit and compensation committees of the Company and the Subsidiaries; and inquiries of certain officials of the Company who have responsibility for financial and accounting matters of the Company and its Subsidiaries as to transactions and events subsequent to December 31, 2004, nothing came to their attention which caused them to believe that:

(1) any unaudited financial statements included or incorporated by reference in the Registration Statement and the Final Prospectus do not comply as to form in all material respects with applicable accounting requirements of the Act and with the related rules and regulations adopted by the Commission with respect to financial statements included or incorporated by reference in quarterly reports on Form 10-Q under the Exchange Act; and said unaudited financial statements are not in conformity with generally accepted accounting principles applied on a basis substantially consistent with that of the audited financial statements included or incorporated by reference in the Registration Statement and the Final Prospectus;

(2) with respect to the period subsequent to June 30, 2005, there were any changes, at a specified date not more than five days prior to the date of the letter, in the long-term debt of the Company and its Subsidiaries or capital stock of the Company or decreases in the stockholders’ equity of the Company or decreases in total assets, cash or cash equivalents or working capital as compared with the amounts shown on the June 30, 2005 consolidated balance sheet included or incorporated by reference in the Registration Statement and the Final Prospectus, or for the period from July 1, 2005, to such specified date there were any decreases, as compared with the corresponding period in the preceding year, in product and service revenue or research and development contract revenue, or increases, as compared with the corresponding period in the preceding year, in net loss or loss per share of the Company and its Subsidiaries, except in all instances for changes or decreases set forth in such letter, in which case the letter shall be accompanied by an explanation by the Company as to the significance thereof unless said explanation is not deemed necessary by the Representatives;

(3) the information included or incorporated by reference in the Registration Statement and Final Prospectus in response to Regulation S-K, Item 301 (Selected Financial Data), Item 302 (Supplementary Financial Information) and Item 402 (Executive Compensation) is not in conformity with the applicable disclosure requirements of Regulation S-K;

(iii) they have performed certain other specified procedures as a result of which they determined that certain information of an accounting, financial or statistical nature (which is limited to accounting, financial or statistical information derived from the general accounting records of the Company and its Subsidiaries) set forth in the Registration Statement and the Final Prospectus and in Exhibit 12 to the Registration Statement, including the information set forth under the captions “Selected Historical Consolidated Financial Data” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” in the Final Prospectus, the information included or incorporated by reference in Items 1, 2, 3, 5, 6, 7, 11 and 12 of the Company’s Annual Report on

Form 10-K, incorporated by reference in the Registration Statement and the Final Prospectus, and the information included in the “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included or incorporated by reference in the Company’s Quarterly Reports on Form 10-Q, incorporated by reference in the Registration Statement and the Final Prospectus, agrees with the accounting records of the Company and its Subsidiaries, excluding any questions of legal interpretation.

References to the Final Prospectus in this paragraph (e) include any supplement thereto at the date of the letter.

(f) Subsequent to the Execution Time or, if earlier, the dates as of which information is given in the Registration Statement (exclusive of any amendment thereof) and the Final Prospectus (exclusive of any supplement thereto), there shall not have been (i) any change or decrease specified in the letter or letters referred to in paragraph (e) of this Section 6 or (ii) any change, or any development involving a prospective change, in or affecting the condition (financial or otherwise), earnings, business or properties of the Company and its Subsidiaries, taken as a whole, whether or not arising from transactions in the ordinary course of business, except as set forth in or contemplated in the Final Prospectus (exclusive of any supplement thereto) the effect of which, in any case referred to in clause (i) or (ii) above, is, in the sole judgment of the Representatives, so material and adverse as to make it impractical or inadvisable to proceed with the offering or delivery of the Securities as contemplated by the Registration Statement (exclusive of any amendment thereof) and the Final Prospectus (exclusive of any supplement thereto).

(g) Prior to the Closing Date, the Company shall have furnished to the Representatives such further information, certificates and documents as the Representatives may reasonably request.

(h) Subsequent to the Execution Time, there shall not have been any decrease in the rating of any of the Company’s debt securities by any “nationally recognized statistical rating organization” (as defined for purposes of Rule 436(g) under the Act) or any notice given of any intended or potential decrease in any such rating or of a possible change in any such rating that does not indicate the direction of the possible change.

(i) At the Execution Time, the Company shall have furnished to the Representatives a letter substantially in the form of Exhibit A hereto from each officer and director addressed to the Representatives.

If any of the conditions specified in this Section 6 shall not have been fulfilled when and as provided in this Agreement, or if any of the opinions and certificates mentioned above or elsewhere in this Agreement shall not be in reasonably satisfactory in form and substance to the Representatives and counsel for the Underwriters, this Agreement and all obligations of the Underwriters hereunder may be canceled at, or at any time prior to, the Closing

Date by the Representatives. Notice of such cancellation shall be given to the Company in writing or by telephone or facsimile confirmed in writing.

The documents required to be delivered by this Section 6 shall be delivered at the office of King & Spalding LLP, counsel for the Underwriters, at 0000 Xxxxxx xx xxx Xxxxxxxx, Xxx Xxxx, Xxx Xxxx 00000, on the Closing Date.

7. Reimbursement of Underwriters’ Expenses. If the sale of the Securities provided for herein is not consummated because any condition to the obligations of the Underwriters set forth in Section 6 hereof is not satisfied, because of any termination pursuant to Section 10(i)(A) hereof or because of any refusal, inability or failure on the part of the Company to perform any agreement herein or comply with any provision hereof other than by reason of a default by any of the Underwriters, the Company will reimburse the Underwriters severally through Citigroup Global Markets Inc. on demand for all out-of-pocket expenses (including reasonable fees and disbursements of counsel) that shall have been incurred by them in connection with the proposed purchase and sale of the Securities.

8. Indemnification and Contribution. (a) The Company agrees to indemnify and hold harmless each Underwriter, the directors, officers, employees and agents of each Underwriter and each person who controls any Underwriter within the meaning of either the Act or the Exchange Act against any and all losses, claims, damages or liabilities, joint or several, to which they or any of them may become subject under the Act, the Exchange Act or other Federal or state statutory law or regulation, at common law or otherwise, insofar as such losses, claims, damages or liabilities (or actions in respect thereof) arise out of or are based upon any untrue statement or alleged untrue statement of a material fact contained in the registration statement for the registration of the Securities as originally filed or in any amendment thereof, or in the Basic Prospectus, any Preliminary Final Prospectus or the Final Prospectus, or in any amendment thereof or supplement thereto, or arise out of or are based upon the omission or alleged omission to state therein a material fact required to be stated therein or necessary to make the statements therein not misleading, and agrees to reimburse each such indemnified party, as incurred, for any legal or other expenses reasonably incurred by them in connection with investigating or defending any such loss, claim, damage, liability or action; provided, however, that the Company will not be liable in any such case to the extent that any such loss, claim, damage or liability arises out of or is based upon any such untrue statement or alleged untrue statement or omission or alleged omission made therein in reliance upon and in conformity with written information furnished to the Company by or on behalf of any Underwriter through the Representatives specifically for inclusion therein. This indemnity agreement will be in addition to any liability which the Company may otherwise have.

(b) Each Underwriter severally and not jointly agrees to indemnify and hold harmless the Company, each of its directors, each of its officers who signs the Registration Statement, and each person who controls the Company within the meaning of either the Act or the Exchange Act, to the same extent as the foregoing indemnity from the Company to each Underwriter, but only with reference to written information relating to such Underwriter furnished to the Company by or on behalf of such Underwriter through the Representatives specifically for inclusion in the documents referred to in the foregoing indemnity. This

indemnity agreement will be in addition to any liability which any Underwriter may otherwise have. The Company acknowledges that the statements set forth in the last paragraph of the cover page regarding delivery of the Securities and, under the heading “Underwriting” or “Plan of Distribution”, (i) the list of Underwriters and their respective participation in the sale of the Securities, (ii) the sentences related to concessions and reallowances and (iii) the paragraph related to stabilization, syndicate covering transactions and penalty bids in any Preliminary Final Prospectus and the Final Prospectus constitute the only information furnished in writing by or on behalf of the several Underwriters for inclusion in any Preliminary Final Prospectus or the Final Prospectus.

(c) Promptly after receipt by an indemnified party under this Section 8 of notice of the commencement of any action, such indemnified party will, if a claim in respect thereof is to be made against the indemnifying party under this Section 8, notify the indemnifying party in writing of the commencement thereof; but the failure so to notify the indemnifying party (i) will not relieve it from liability under paragraph (a) or (b) above unless and to the extent it did not otherwise learn of such action and such failure results in the forfeiture by the indemnifying party of substantial rights and defenses and (ii) will not, in any event, relieve the indemnifying party from any obligations to any indemnified party other than the indemnification obligation provided in paragraph (a) or (b) above. The indemnifying party shall be entitled to appoint counsel of the indemnifying party’s choice at the indemnifying party’s expense to represent the indemnified party in any action for which indemnification is sought (in which case the indemnifying party shall not thereafter be responsible for the fees and expenses of any separate counsel retained by the indemnified party or parties except as set forth below); provided, however, that such counsel shall be satisfactory to the indemnified party. Notwithstanding the indemnifying party’s election to appoint counsel to represent the indemnified party in an action, the indemnified party shall have the right to employ separate counsel (including local counsel), and the indemnifying party shall bear the reasonable fees, costs and expenses of such separate counsel if (i) the use of counsel chosen by the indemnifying party to represent the indemnified party would present such counsel with a conflict of interest, (ii) the actual or potential defendants in, or targets of, any such action include both the indemnified party and the indemnifying party and the indemnified party shall have reasonably concluded that there may be legal defenses available to it and/or other indemnified parties which are different from or additional to those available to the indemnifying party, (iii) the indemnifying party shall not have employed counsel satisfactory to the indemnified party to represent the indemnified party within a reasonable time after notice of the institution of such action or (iv) the indemnifying party shall authorize the indemnified party to employ separate counsel at the expense of the indemnifying party. An indemnifying party will not, without the prior written consent of the indemnified parties, settle or compromise or consent to the entry of any judgment with respect to any pending or threatened claim, action, suit or proceeding in respect of which indemnification or contribution may be sought hereunder (whether or not the indemnified parties are actual or potential parties to such claim or action) unless such settlement, compromise or consent includes an unconditional release of each indemnified party from all liability arising out of such claim, action, suit or proceeding.

(d) In the event that the indemnity provided in paragraph (a), (b) or (c) of this Section 8 is unavailable to or insufficient to hold harmless an indemnified party for any reason, the Company and the Underwriters severally agree to contribute to the aggregate losses, claims,

damages and liabilities (including legal or other expenses reasonably incurred in connection with investigating or defending same) (collectively “Losses”) to which the Company and one or more of the Underwriters may be subject in such proportion as is appropriate to reflect the relative benefits received by the Company on the one hand and by the Underwriters on the other from the offering of the Securities; provided, however, that in no case shall (i) any Underwriter (except as may be provided in any agreement among underwriters relating to the offering of the Securities) be responsible for any amount in excess of the underwriting discount or commission applicable to the Securities purchased by such Underwriter hereunder. If the allocation provided by the immediately preceding sentence is unavailable for any reason, the Company and the Underwriters severally shall contribute in such proportion as is appropriate to reflect not only such relative benefits but also the relative fault of the Company on the one hand and of the Underwriters on the other in connection with the statements or omissions which resulted in such Losses as well as any other relevant equitable considerations. Benefits received by the Company shall be deemed to be equal to the total net proceeds from the offering (before deducting expenses) received by it, and benefits received by the Underwriters shall be deemed to be equal to the total underwriting discounts and commissions, in each case as set forth on the cover page of the Final Prospectus. Relative fault shall be determined by reference to, among other things, whether any untrue or any alleged untrue statement of a material fact or the omission or alleged omission to state a material fact relates to information provided by the Company on the one hand or the Underwriters on the other, the intent of the parties and their relative knowledge, access to information and opportunity to correct or prevent such untrue statement or omission. The Company and the Underwriters agree that it would not be just and equitable if contribution were determined by pro rata allocation or any other method of allocation which does not take account of the equitable considerations referred to above. Notwithstanding the provisions of this paragraph (d), no person guilty of fraudulent misrepresentation (within the meaning of Section 11(f) of the Act) shall be entitled to contribution from any person who was not guilty of such fraudulent misrepresentation. For purposes of this Section 8, each person who controls an Underwriter within the meaning of either the Act or the Exchange Act and each director, officer, employee and agent of an Underwriter shall have the same rights to contribution as such Underwriter, and each person who controls the Company within the meaning of either the Act or the Exchange Act, each officer of the Company who shall have signed the Registration Statement and each director of the Company shall have the same rights to contribution as the Company, subject in each case to the applicable terms and conditions of this paragraph (d).

9. Default by an Underwriter. If any one or more Underwriters shall fail to purchase and pay for any of the Securities agreed to be purchased by such Underwriter or Underwriters hereunder and such failure to purchase shall constitute a default in the performance of its or their obligations under this Agreement, the remaining Underwriters shall be obligated severally to take up and pay for (in the respective proportions which the amount of Securities set forth opposite their names in Schedule II hereto bears to the aggregate amount of Securities set forth opposite the names of all the remaining Underwriters) the Securities which the defaulting Underwriter or Underwriters agreed but failed to purchase; provided, however, that in the event that the aggregate amount of Securities which the defaulting Underwriter or Underwriters agreed but failed to purchase shall exceed 10% of the aggregate amount of Securities set forth in Schedule II hereto, the remaining Underwriters shall have the right to purchase all, but shall not be under any obligation to purchase any, of the Securities, and if such nondefaulting

Underwriters do not purchase all the Securities, this Agreement will terminate without liability to any nondefaulting Underwriter or the Company. In the event of a default by any Underwriter as set forth in this Section 9, the Closing Date shall be postponed for such period, not exceeding five Business Days, as the Representatives shall determine in order that the required changes in the Registration Statement and the Final Prospectus or in any other documents or arrangements may be effected. Nothing contained in this Agreement shall relieve any defaulting Underwriter of its liability, if any, to the Company and any nondefaulting Underwriter for damages occasioned by its default hereunder.

10. Termination. This Agreement shall be subject to termination in the absolute discretion of the Representatives, by notice given to the Company prior to delivery of and payment for the Securities, if at any time prior to such time (i) (A) trading in the Company’s Common Stock shall have been suspended by the Commission or Nasdaq or (B) trading in securities generally on the New York Stock Exchange or Nasdaq shall have been suspended or limited or minimum prices shall have been established on such exchange or Nasdaq, (ii) a banking moratorium shall have been declared either by Federal or New York State authorities or (iii) there shall have occurred any outbreak or escalation of hostilities, declaration by the United States of a national emergency or war, or other calamity or crisis the effect of which on financial markets is such as to make it, in the sole judgment of the Representatives, impractical or inadvisable to proceed with the offering or delivery of the Securities as contemplated by the Final Prospectus (exclusive of any supplement thereto).

11. Representations and Indemnities to Survive. The respective agreements, representations, warranties, indemnities and other statements of the Company or its officers and of the Underwriters set forth in or made pursuant to this Agreement will remain in full force and effect, regardless of any investigation made by or on behalf of any Underwriter or the Company or any of the officers, directors, employees, agents or controlling persons referred to in Section 8 hereof, and will survive delivery of and payment for the Securities. The provisions of Sections 7 and 8 hereof shall survive the termination or cancellation of this Agreement.

12. Notices. All communications hereunder will be in writing and effective only on receipt, and, if sent to any Underwriter will be mailed, delivered or telefaxed and confirmed to it at its address set forth on Schedule I with copies to: Citigroup Global Markets Inc., General Counsel (fax no.: (000) 000-0000), Xxxxxxxx Inc., General Counsel (fax no.: (000) 000-0000) and King & Spalding LLP, Xxxx X. Xxxxxx III, Esq. (fax no.: (000) 000-0000) or, if sent to the Company, will be mailed, delivered or telefaxed to Chief Executive Officer (fax no.: (000) 000-0000) and confirmed to it at 000 Xxxxxx-Xxxxxx Xxxx, Xxxxxx, Xxx Xxxx 00000 Attention: Legal Department with a copy to Xxxxxxx Procter LLP, Xxxxxx X. Xxxxxx, Xx. (fax no.: (000) 000-0000).

13. Successors. This Agreement will inure to the benefit of and be binding upon the parties hereto and their respective successors and the officers, directors, employees, agents and controlling persons referred to in Section 8 hereof, and no other person will have any right or obligation hereunder.

14. Applicable Law. This Agreement will be governed by and construed in accordance with the laws of the State of New York applicable to contracts made and to be

performed within the State of New York. The Company hereby submits to the nonexclusive jurisdiction of the Federal and state courts in the Borough of Manhattan in the City of New York in any suit or proceeding arising out of or relating to this Agreement or the transactions contemplated hereby.

15. No Fiduciary Relationship. The Company hereby acknowledges that (a) each Underwriter is acting as principal and not as an agent or fiduciary of the Company and (b) its engagement of the Underwriters in connection with the offering of the Securities is as independent contractors and not in any other capacity. Furthermore, the Company agrees that it is solely responsible for making its own judgments in connection with the offering of the Securities (irrespective of whether the Underwriters have advised or are currently advising the Company on related or other matters).

16. Counterparts. This Agreement may be signed in one or more counterparts, each of which shall constitute an original and all of which together shall constitute one and the same agreement.

17. Headings. The section headings used herein are for convenience only and shall not affect the construction hereof.

18. Definitions. The terms which follow, when used in this Agreement, shall have the meanings indicated.

“Act” shall mean the Securities Act of 1933, as amended, and the rules and regulations of the Commission promulgated thereunder.

“Basic Prospectus” shall mean the prospectus referred to in paragraph 1(a) above contained in the Registration Statement at the Effective Date including any Preliminary Final Prospectus.

“Business Day” shall mean any day other than a Saturday, a Sunday or a legal holiday or a day on which banking institutions or trust companies are authorized or obligated by law to close in New York City.

“Commission” shall mean the Securities and Exchange Commission.

“Effective Date” shall mean each date and time that the Registration Statement, any post-effective amendment or amendments thereto and any Rule 462(b) Registration Statement became or become effective.

“Exchange Act” shall mean the Securities Exchange Act of 1934, as amended, and the rules and regulations of the Commission promulgated thereunder.

“Execution Time” shall mean the date and time that this Agreement is executed and delivered by the parties hereto.

“Final Prospectus” shall mean the prospectus supplement relating to the Securities that was first filed pursuant to Rule 424(b) after the Execution Time, together with the Basic Prospectus.

“FCPA” means Foreign Corrupt Practices Act of 1977, as amended, and the rules and regulations thereunder.

“Preliminary Final Prospectus” shall mean any preliminary prospectus supplement to the Basic Prospectus which describes the Securities and the offering thereof and is used prior to filing of the Final Prospectus, together with the Basic Prospectus.

“Registration Statement” shall mean the registration statement referred to in paragraph 1(a) above, including exhibits and financial statements, as amended at the Execution Time (or, if not effective at the Execution Time, in the form in which it shall become effective) and, in the event any post-effective amendment thereto or any Rule 462(b) Registration Statement becomes effective prior to the Closing Date, shall also mean such registration statement as so amended or such Rule 462(b) Registration Statement, as the case may be. Such term shall include any Rule 430A Information deemed to be included therein at the Effective Date as provided by Rule 430A.

“Rule 415”, “Rule 424”, “Rule 430A” and “Rule 462” refer to such rules under the Act.

“Rule 430A Information” shall mean information with respect to the Securities and the offering thereof permitted to be omitted from the Registration Statement when it becomes effective pursuant to Rule 430A.

“Rule 462(b) Registration Statement” shall mean a registration statement and any amendments thereto filed pursuant to Rule 462(b) relating to the offering covered by the registration statement referred to in Section 1(a) hereof.

“Subsidiaries” shall mean H Power Corporation, a Delaware corporation, and Plug Power Holding Inc., a Delaware corporation.

If the foregoing is in accordance with your understanding of our agreement, please sign and return to us the enclosed duplicate hereof, whereupon this letter and your acceptance shall represent a binding agreement among the Company and the several Underwriters.


Very truly yours,

PLUG POWER INC.

By:		/S/ XXXXX X. XXXXXXX
		Name: Xxxxx X. Xxxxxxx Title: Chief Financial Officer

The foregoing Agreement is

hereby confirmed and accepted

as of the date specified in

Schedule I hereto.

Citigroup Global Markets Inc.




By:		/S/ XXXXX XXXXX
		Name: Xxxxx Xxxxx Title: Director

For itself and the other

Underwriters named in

Schedule II to the

foregoing Agreement.

SCHEDULE I

Underwriting Agreement dated August 4, 2005

Representative(s): Citigroup Global Markets, Inc., as Sole-Book Runner

Title, Purchase Price and Description of Securities:

Title: Common Stock

Number of Shares to be sold by the Company: 11,000,000

Price to Public per Share (include accrued dividends, if any): $6.25

Price to Public — total: $68,750,000

Underwriting Discount per Share: 0.3438

Underwriting Discount — total: $3,781,800

Proceeds to Company per Share: $5.9062

Proceeds to Company — total: $64,968,200

Other provisions: N/A

Closing Date, Time and Location:

August 10, 2005 at 10:00 a.m. at King & Spalding LLP,

0000 Xxxxxx xx xxx Xxxxxxxx, Xxx Xxxx, Xxx Xxxx

00000.

Type of Offering: Non-Delayed

Date referred to in Section 5(f) after which the Company may offer or sell securities issued or guaranteed by the Company without the consent of the Representative(s): 90 days from the date of this Agreement.

Date referred to in Section 5(g) after which the Company is no longer required to consent (pursuant to this Agreement) to sales of shares of Common Stock by its officers: 90 days from the date of this Agreement.

Modification of items to be covered by the letter from KPMG LLP delivered pursuant to Section 6(e) at the Execution Time: N/A

SCHEDULE II


Underwriters		Number of Underwritten Securities to be Purchased
Citigroup Global Markets Inc.		7,150,000
Xxxxxxxx Inc.		3,850,000

Total		11,000,000

Option Securities:		up to: 1,650,000 Share

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Plug Power Inc. 11,000,000 Shares Common Stock ($.01 par value) Underwriting Agreement

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