December 31, 2008 PERSONAL & CONFIDENTIAL

Exhibit 10.18

December 31, 2008

PERSONAL & CONFIDENTIAL

C. Xxxxxx Xxxxxxxxxx

[Home address redacted]

Re: Employment Agreement dated January 4, 2008 between you and NYFIX, Inc.

(the “Agreement”).

Dear Xxx:

You and NYFIX, Inc. agree to the following amendments to the Agreement.

1. The sixth sentence of the last full paragraph (such paragraph beginning with the words “While we certainly hope”) beginning on Page 4 of your agreement (such sentence beginning with the words “Notwithstanding the above, in the event you terminate your employment for ‘Good Reason’”) is amended as follows:

a. The words “provided you sign a Release containing generally the release language attached as Attachment E to this Agreement (the “Release”)” are deleted;

b. The words ” , subject to Attachment C,” are inserted prior to the words “(i) twelve (12) months’ Base Salary”;

c. Subparagraph (i) of such sentence is amended by adding the words “and commencing 30 days following the termination of employment” prior to the semicolon;

d. Subparagraph (ii) of such sentence is amended by adding the words “payable in a lump sum within 30 days of your termination” prior to the semicolon;

e. The words “but not later than March 15 of the applicable following year” appearing at the end of such sentence are deleted and replaced by the words “but not later than March 15 of the year following termination.”

2. The seventh sentence of the last full paragraph beginning on Page 4 of your agreement (such sentence beginning with the words “Alternatively, in the event you terminate your employment for Good Reason”) is amended as follows:

Page 1 of 5

a. The words “payable in accordance with the Company’s normal payroll practices and commencing 30 days following termination of your employment,” are inserted before the words “less required withholdings”;

b. The words “provided you execute a Release containing generally the release language attached as Attachment E to this Agreement” appearing after the words “less required withholdings” are deleted.

3. A new (eighth) sentence is inserted after the seventh sentence of such paragraph, to read in full as follows:

“Your severance payments and benefits provided above shall be made or shall commence according to the terms of this Agreement on the 30th day after your termination, provided that you execute a release containing the general release language attached as Attachment E to this Agreement (the “Release”), and that Release has become binding and any applicable revocation period has lapsed as of such date.”

4. A new (tenth) sentence is inserted at the end of such paragraph, to read in full as follows:

“The distribution of the payments and benefits you receive upon your termination of employment with the Company shall be subject to the terms and conditions set forth in Attachment C.”

5. Attachment C is amended to read in its entirety as set forth in the attachment hereto.

If you are in agreement with the foregoing, kindly execute a copy of this letter and return it to the undersigned.

Page 2 of 5

NYFIX, Inc.

Very truly yours,

/s/ Xxxxxx Xxxxxxxxx

Xxxxxx Xxxxxxxxx

Chief Financial Officer

Accepted and Agreed:

/s/ C. Xxxxxx Xxxxxxxxxx

C. Xxxxxx Xxxxxxxxxx

December 31, 2008

Page 3 of 5

Attachment C

Payments Subject to Section 409A

The parties mutually agree to cooperate on the application of 409A to the agreement.  Subject to the provisions in this Attachment C, severance payments or benefits under this offer letter shall begin only upon the date of your “separation from service” (determined as set forth below) which occurs on or after the date of termination of employment.  The following rules shall apply with respect to distribution of the payments and benefits, if any, to be provided to you under this offer letter:

(a) It is intended that each installment of the severance payments and benefits provided under this offer letter shall be treated as a separate “payment” for purposes of Section 409A of the Internal Revenue Code and the guidance issued thereunder (“Section 409A”).  Neither you nor the Company shall have the right to accelerate or defer the delivery of any such payments or benefits except to the extent specifically permitted or required by Section 409A.

(b) If, as of the date of your “separation from service” from the Company, you are not a “specified employee” (within the meaning of Section 409A), then each installment of the severance payments and benefits shall be made on the dates and terms set forth in this offer letter.

(c) If, as of the date of your  “separation from service” from the Company, you are a “specified employee” (within the meaning of Section 409A), then:

(i) Each installment of the severance payments and benefits due under this offer letter that, in accordance with the dates and terms set forth herein, will in all circumstances, regardless of when the separation from service occurs, be paid within the short-term deferral period (as defined under Section 409A) shall be treated as a short-term deferral within the meaning of Treasury Regulation Section 1.409A-1(b)(4) to the maximum extent permissible under Section 409A; and

(ii) Each installment of the severance payments and benefits due under this offer letter that is not described in paragraph (i) above and that would, absent this subsection, be paid within the six-month period following your “separation from service” from the Company shall not be paid until the date that is six months and one day after such separation from service (or, if earlier, your death), with any such installments that are required to be delayed being accumulated during the six-month period and paid in a lump sum on the date that is six months and one day following your separation from service and any subsequent installments, if any, being paid in accordance with the dates and terms set forth herein; provided, however, that the preceding provisions of this sentence shall not apply to any installment of severance payments and benefits if and to the maximum extent that that such installment is deemed to be paid under a separation pay plan that does not provide for a deferral of compensation by reason of the application of Treasury Regulation 1.409A-1(b)(9)(iii) (relating to separation pay upon an involuntary separation from service).  Any installments that qualify for the exception under Treasury Regulation Section 1.409A-1(b)(9)(iii) must be paid no later than the last day of your second taxable year following the taxable year in which the separation from service occurs.

Page 4 of 5

(d) The determination of whether and when your separation from service from the Company has occurred shall be made and in a manner consistent with, and based on the presumptions set forth in, Treasury Regulation Section 1.409A-1(h).  Solely for purposes of this paragraph (d), “Company” shall include all persons with whom the Company would be considered a single employer as determined under Treasury Regulation Section 1.409A-(h)(3).

(e) All reimbursements and in-kind benefits provided under this offer letter shall be made or provided in accordance with the requirements of Section 409A to the extent that such reimbursements or in-kind benefits are subject to Section 409A, including, where applicable, the requirement that (i) any reimbursement is for expenses incurred during your lifetime (or during a shorter period of time specified in this offer letter), (ii) the amount of expenses eligible for reimbursement during a calendar year may not affect the expenses eligible for reimbursement in any other calendar year, (iii) the reimbursement of an eligible expense will be made on or before the last day of the calendar year following the year in which the expense is incurred and (iv) the right to reimbursement is not subject to set off or liquidation or exchange for any other benefit.

(f) The Company may withhold (or cause to be withheld) from any payments made under this offer letter, all federal, state, city or other taxes as shall be required to be withheld pursuant to any law or governmental regulation or ruling.

Page 5 of 5