EXHIBIT31.1

CERTIFICATION

I, Alan Kessman, ChiefExecutive Officer of Vion Pharmaceuticals, Inc., certify that:

1.    I have reviewed this annual report on Form 10-K of VionPharmaceuticals, Inc. (the "Registrant");

2.    Based on my knowledge, this annual report does not containany untrue statement of a material fact or omit to state a materialfact necessary to make the statements made, in light of thecircumstances under which such statements were made, not misleadingwith respect to the period covered by this annual report;

3.    Based on my knowledge, the financial statements, and otherfinancial information included in this annual report, fairly present inall material respects the financial condition, results of operationsand cash flows of the registrant as of, and for, the periods presentedin this annual report;

4.    The Registrant's othercertifying officer and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in ExchangeAct Rules 13a-15(e) and 15d-15(e)) for the Registrant and have:

a)    Designed such disclosure controls andprocedures, or caused such disclosure controls and procedures to bedesigned under our supervision, to ensure that material informationrelating to the Registrant, including its consolidated subsidiaries, ismade known to us by others within those entities, particularly duringthe period in which this annual report is being prepared;

b)    Evaluated the effectiveness of theRegistrant's disclosure controls and procedures and presented inthis annual report our conclusions about the effectiveness of thedisclosure controls and procedures, as of the end of the period coveredby this annual report based on such evaluation;

c)    Designed such internal control overfinancial reporting, or caused such internal control over financialreporting to be designed under our supervision, to provide reasonableassurance regarding the reliability of financial reporting and thepreparation of financial statements for external purposes in accordancewith generally accepted accounting principles;

d)    Disclosed in this annual report any changein the Registrant's internal control over financial reportingthat occurred during the Registrant's most recent fiscal quarter(the Registrant's fourth fiscal quarter) that has materiallyaffected, or is reasonably likely to materially affect, theRegistrant's internal control over financial reporting; and

5.    The Registrant's other certifying officer and I havedisclosed, based on our most recent evaluation of internal control overfinancial reporting, to the Registrant's auditors and the auditcommittee of Registrant's board of directors (or personsperforming the equivalent functions):

a)    All significant deficiencies and materialweaknesses in the design or operation of internal control overfinancial reporting which are reasonably likely to adversely affect theRegistrant's ability to record, process, summarize and reportfinancial information; and

b)    Any fraud,whether or not material, that involves management or other employeeswho have a significant role in the Registrant's internal controlover financial reporting.

Date: March 29, 2004

|  | /s/ Alan Kessman Alan KessmanChief Executive Officer |
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