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EXHIBIT 31.1
 CERTIFICATIONS REQUIRED BY
RULE 13a-14(a) UNDER THE SECURITIES EXCHANGE ACT OF 1934
  1.
    
    I have reviewed this Form 10-Q of S1 Corporation;2.
    
    Based on my knowledge, this report does not contain any untrue
statement of a material fact or omit to state a material fact
necessary to make the statements made, in light of the circumstances
under which such statements were made, not misleading with respect to
the period covered by this report;3.
    
    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material
respects the financial condition, results of operations and cash flows
of the registrant as of, and for, the periods presented in this
report;4.
    
    The registrants other certifying officers and I are responsible for
establishing and maintaining disclosure controls and procedures (as
defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) and internal
control over financial reporting (as defined in Exchange Act Rules
13a-15(f) and 15d-15(f)) for the registrant and have:
 a)
    
    designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our
supervision, to ensure that material information relating to the
registrant, including its consolidated subsidiaries, is made known to
us by others within those entities, particularly during the period in
which this report is being prepared;b)
    
    designed such internal control over financial reporting, or caused
such internal control over financial reporting to be designed under
our supervision to provide reasonable assurance regarding the
reliability of financial reporting and the preparation of financial
statements for external purposes in accordance with generally accepted
accounting principles;c)
    
    evaluated the effectiveness of the registrants disclosure controls
and procedures and presented in this report our conclusions about the
effectiveness of the disclosure controls and procedures, as of the end
of the period covered by this report based on such evaluation; andd)
    
    disclosed in this report any change in the registrants internal
control over financial reporting that occurred during the registrants
most recent fiscal quarter (the registrants fourth fiscal quarter in
the case of an annual report) that has materially affected, or is
reasonably likely to materially affect, the registrants internal
control over financial reporting,
 5.
    
    The registrants other certifying officers and I have disclosed, based
on our most recent evaluation of internal control over financial
reporting, to the registrants auditors and the audit committee of the
registrants board of directors (or persons performing the equivalent
function):
 a)
    
    all significant deficiencies and material weaknesses in the design or
operation of internal control over financial reporting which are
reasonably likely to adversely affect the registrants ability to
record, process, summarize and report financial information; andb)
    
    any fraud, whether or not material, that involves management or other
employees who have a significant role in the registrants internal
control over financial reporting.
  /s/ James S. Mahan, IIIJames S. Mahan, IIIChief Executive Officer