YPF Sociedad Anónima Av. R.S. Peña 777 Ciudad de Buenos Aires, Agentina, C1035AAC

Exhibit 99.15

YPF Xxxxxxxx Xxxxxxx

Xx. X.X. Xxxx 000

Xxxxxx xx Xxxxxx Xxxxx, Xxxxxxxx, C1035AAC

February 21, 2008

Repsol YPF, S.A.

Xxxxx xx xx Xxxxxxxxxx 000

00000 Xxxxxx

Xxxxx

Xxxxxxxx Energia PTY, Ltd

Xxxxx 00, Xxxxxx Xxxxxx 000

Xxxxxxx Xxxxxx

Xxxxxxxxx, Xxxxxxxxx

Ladies and Gentlemen:

Reference is made to that certain Registration Rights Agreement, dated as of February 21, 2008, among YPF Sociedad Anónima, an Argentine company (“YPF”), Repsol YPF, S.A., a Spanish company (“Repsol”), Xxxxxxxx Energía S.A., a Spanish special purpose company, and certain other parties named therein (the “Agreement”). Capitalized terms used herein but not otherwise defined have the respective meanings assigned to them in the Agreement.

Repsol and Xxxxxxxx Energia PTY Ltd (“Xxxxxxxx”) hereby agree, without limitation as to time, to indemnify to the fullest extent permitted by law, YPF for any Losses paid by it pursuant to Section 7(a) of the Agreement; provided, however, that (i) Repsol and Xxxxxxxx shall only indemnify YPF for any such Losses if a court of competent jurisdiction determines, in a final, non-appealable order, that such Losses were not incurred as a result of a material misstatement or omission contained in the Registration Statement or Prospectus; and (ii) Repsol and Xxxxxxxx shall not indemnify YPF for any such Losses incurred by YPF to the extent YPF makes any payment related to such Losses pursuant to a settlement entered into by YPF before a final, non-appealable, determination of a court of competent jurisdiction; and provided further, that (i) Xxxxxxxx’x obligations in respect of any Loss required to be indemnified hereunder is limited to 14.9% of the total amounts to be indemnified by Repsol and Xxxxxxxx with respect to such Loss (the “Xxxxxxxx Cap”); and (ii) Repsol’s obligations in respect of any Loss required to be indemnified hereunder is limited to 85.1% of the total amounts to be indemnified by Repsol and Xxxxxxxx with respect to such loss (the “Repsol Cap”). In the event that Xxxxxxxx and one or more of its affiliates acquires Option Securities pursuant to any Option Agreement from time to time, the Xxxxxxxx Cap

with respect to any Losses incurred as a result of claims brought against YPF after the date of such increase shall be adjusted upwards to an amount equal to the percentage of total outstanding YPF shares (including for purposes of this calculation any shares represented by ADSs) held by Xxxxxxxx and its affiliates, and the Repsol Cap shall be adjusted downwards to an amount equal to the percentage of total outstanding YPF shares (including any shares represented by ADSs) held by Repsol (including for purposes of this calculation any shares (including shares represented by ADS) transferred by Repsol after the date hereof to any party other than Option Securities acquired by Xxxxxxxx and its affiliates). The Xxxxxxxx Cap and the Repsol Cap in effect at the time an indemnity claim is made under the Agreement shall apply to any Losses for which Xxxxxxxx and Repsol indemnify YPF hereunder.

This letter agreement shall be governed by and construed in accordance with the law of the State of New York, without regard to the conflicts of law rules of such state.

Please sign in the space indicated below to acknowledge your agreement with the above.

[Signature pages follow]

Very truly yours,

YPF Sociedad AnÓnima
By:	/s/ Antonio Xxxxx Xxxx
	Name: Antonio Xxxxx Xxxx Title:   Executive Director

Acknowledged and Agreed

as of the date written above:

Repsol YPF, S.A.

By:

Name:
Title:

Xxxxxxxx Energia PTY LTD
By:	/s/ Xxxxxxx Xxxxxx
	Name: Xxxxxxx Xxxxxx Title:  Director - Attorney in Fact