##### 



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, III

James S. Mahan, IIIChief Executive Officer