**EXHIBIT 31.1**
**CERTIFICATIONS REQUIRED BY****RULE 13a-14(a) UNDER THE SECURITIES EXCHANGE ACT OF 1934**
I, Jaime W. Ellertson, certify that:
1.  I have reviewed this Form 10-Q of S1 Corporation; 2.  Based on my knowledge, this report does not contain anyuntrue statement of a material fact or omit to state amaterial fact necessary to make the statements made, in lightof the circumstances under which such statements were made,not misleading with respect to the period covered by thisreport; 3.  Based on my knowledge, the financial statements, and otherfinancial information included in this report, fairly presentin all material respects the financial condition, results ofoperations 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 areresponsible for establishing and maintaining disclosurecontrols and procedures (as defined in Exchange Act Rules13a-15(e) and 15d-15(e)) and internal control over financialreporting (as defined in Exchange Act Rules 13a-15(f) and15d-15(f)) for the registrant and have:
 a)  designed such disclosurecontrols and procedures, orcaused such disclosurecontrols and procedures to bedesigned under oursupervision, to ensure thatmaterial information relatingto the registrant, includingits consolidated subsidiaries,is made known to us by otherswithin those entities,particularly during the periodin which this report is beingprepared;  b)  designed such internal controlover financial reporting, orcaused such internal controlover financial reporting to bedesigned under our supervisionto provide reasonableassurance regarding thereliability of financialreporting and the preparationof financial statements forexternal purposes inaccordance with generallyaccepted accountingprinciples;  c)  evaluated the effectiveness ofthe registrant’s disclosurecontrols and procedures andpresented in this report ourconclusions about theeffectiveness of thedisclosure controls andprocedures, as of the end ofthe period covered by thisreport based on suchevaluation; and  d)  disclosed in this report anychange in the registrant’sinternal control overfinancial reporting thatoccurred during theregistrant’s most recentfiscal quarter (theregistrant’s fourth fiscalquarter in the case of anannual report) that hasmaterially affected, or isreasonably likely tomaterially affect, theregistrant’s internal controlover financial reporting,
5.  The registrant’s other certifying officers and I havedisclosed, based on our most recent evaluation of internalcontrol over financial reporting, to the registrant’sauditors and the audit committee of the registrant’s board ofdirectors (or persons performing the equivalent function):
 a)  all significant deficienciesand material weaknesses in thedesign or operation ofinternal control overfinancial reporting which arereasonably likely to adverselyaffect the registrant’sability to record, process,summarize and report financialinformation; and  b)  any fraud, whether or notmaterial, that involvesmanagement or other employeeswho have a significant role inthe registrant’s internalcontrol over financialreporting.
Date: May 9, 2005   /s/ Jaime W. Ellertson  Jaime W. Ellertson  Chief Executive Officer