EXHIBIT 32.1


                  CERTIFICATION OF THE CHIEF EXECUTIVE OFFICER
          PURSUANT TO RULE 13a-14(b) OF THE EXCHANGE ACT AND 18 U.S.C.
                                  SECTION 1350.

     In connection with the Annual Report on Form 10-K of Penwest
Pharmaceuticals Co. (the "Company") for the fiscal year ended December 31, 2004
as filed with the Securities and Exchange Commission on the date hereof (the
"Report"), the undersigned, Robert J. Hennessey, President and Chief Executive
Officer of the Company, hereby certifies, 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.

Dated:  March 14, 2005
                                          /s/ Robert J. Hennessey
                                          --------------------------------------
                                          Robert J. Hennessey
                                          President and Chief Executive Officer