Exhibit 99.1

                            CERTIFICATION PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002


In  connection  with the  Quarterly  Report of Valero  Energy  Corporation  (the
Company) on Form 10-Q for the quarter  ending March 31, 2003,  as filed with the
Securities and Exchange  Commission on the date hereof (the Report),  I, William
E. Greehey, Chief Executive Officer of the Company, hereby certify,  pursuant to
18 U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley
Act of 2002, that:

(1) The Report fully complies with the requirements of Section 13(a) or 15(d) of
the Securities Exchange Act of 1934; and

(2) The  information  contained in the Report fairly  presents,  in all material
respects, the financial condition and results of operations of the Company.



 /s/ William E. Greehey
- -----------------------
William E. Greehey
Chief Executive Officer
May 15, 2003

A signed  original  of the  written  statement  required by Section 906 has been
provided to Valero  Energy  Corporation  and will be  retained by Valero  Energy
Corporation and furnished to the Securities and Exchange Commission or its staff
upon request.