EXHIBIT31.1

CERTIFICATION

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

1.    I have reviewed this amendment to annual report on Form10-K/A (the "Amendment to Annual Report") ofVion Pharmaceuticals, Inc. (the"Registrant");

2.    Based on myknowledge, this Amendment to Annual Report does not contain any untruestatement of a material fact or omit to state a material fact necessaryto make the statements made, in light of the circumstances under whichsuch statements were made, not misleading with respect to the periodcovered by this Amendment to Annual Report;

3.    Based on myknowledge, the financial statements, and other financial informationincluded in this Amendment to Annual Report, fairly present in allmaterial respects the financial condition, results of operations andcash flows of the registrant as of, and for, the periods presented inthis Amendment to Annual Report;

4.    The Registrant'sother certifying 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 Amendment to Annual Report is beingprepared;

b)    Evaluated the effectiveness ofthe Registrant's disclosure controls and procedures and presentedin this Amendment to Annual Report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the endof the period covered by this Amendment to Annual Report based on suchevaluation;

c)    Designed such internalcontrol over financial reporting, or caused such internal control overfinancial reporting to be designed under our supervision, to providereasonable assurance regarding the reliability of financial reportingand the preparation of financial statements for external purposes inaccordance with generally accepted accounting principles;

d)    Disclosed in this Amendment to Annual Reportany change in the Registrant's internal control over financialreporting that occurred during the Registrant's most recentfiscal quarter (the Registrant's fourth fiscal quarter) that hasmaterially affected, 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: May 25, 2004

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