LIMITED POWER OF ATTORNEY FOR
  SECTION 16 REPORTING OBLIGATIONS

            Know all by these presents, that the undersigned hereby makes,
  constitutes and appoints each of Richard Dobb, Esq.; Stuart Stein, Esq.;
  Daniel Keating, Esq.; and Amit Saluja, Esq. as the undersigned's true and
  lawful attorney in fact, with full power and authority as hereinafter
  described on behalf of and in the name, place and stead of the undersigned to:

  (1)       prepare, execute, acknowledge, deliver and file Forms 3, 4,
  and 5 (including any amendments thereto) with respect to the securities of S1
  Corporation, a Delaware corporation (the "Company"), with the United States
  Securities and Exchange Commission, any national securities exchanges and the
  Company, as considered necessary or advisable under Section 16(a) of the
  Securities Exchange Act of 1934 and the rules and regulations promulgated
  thereunder, as amended form time to time (the "Exchange Act");

  (2)       seek or obtain, as the undersigned's representative and on the
  undersigned's behalf, information or transactions in the Company's securities
  from any third party, including brokers, employee benefit plan administrators
  and trustees, and the undersigned hereby authorizes any such person to release
  any such information to the undersigned and approves and ratifies any such
  release of information; and

  (3)       perform any and all other acts which in the discretion of such
  attorney-in-fact are necessary or desirable for and on behalf of the
  undersigned in connection with the foregoing.

  The undersigned acknowledges that:

  (1)       this Power of Attorney authorizes, but does not require, such
  attorney-in-fact to act in their discretion on information provided to such
  attorney-in-fact without independent verification of such information;

  (2)       any documents prepared and/or executed by such attorney-in-fact
  on behalf of the undersigned pursuant to this Power of Attorney will be in
  such form and will contain such information and disclosure as such
  attorney-in-fact, in his or her discretion, deems necessary or desirable;

  (3)       neither the Company nor such attorney-in-fact assumes (i) any
  liability for the undersigned's responsibility to comply with the requirements
  of the Exchange Act, (ii) any liability of the undersigned for any failure to
  comply with such requirements, or (iii) any obligation or liability of the
  undersigned for profit disgorgement under Section 16(b) of the Exchange Act;
  and

  (4)       this Power of Attorney does not relieve the undersigned from
  responsibility for compliance with the undersigned's obligations under the
  Exchange Act, including without limitation the reporting requirements under
  Section 16 of the Exchange Act.

            The undersigned hereby gives and grants the foregoing
  attorney-in-fact full power and authority to do and perform all and every act
  and thing whatsoever requisite, necessary or appropriate to be done in and
  about the foregoing matters as fully to all intents and purposes as the
  undersigned might or could do if present, hereby ratifying all that such
  attorney-in-fact of, for and on behalf of the undersigned, shall lawfully do
  or cause to be done by virtue of this Limited Power of Attorney.

            This Power of Attorney shall remain in full force and effect until
  revoked by the undersigned in a signed writing delivered to such
  attorney-in-fact.

            IN WITNESS WHEREOF, the undersigned has caused this Power of
  Attorney to be executed as of this 6th day of August 2003.

  /s/James S. Mahan, III
  Signature

  James S. Mahan, III
  Print Name

  STATE OF GEORGIA
  COUNTY OF FULTON

            On this 6th day of August 2003 James S. Mahan, III personally
  appeared before me, and acknowledged that he executed the foregoing instrument
  for the purposes therein contained.

            IN WITNESS WHEREOF, I have hereunto set my hand and official seal.

  Jacqueline D. Allison
  Notary Public