Exhibit 32.1

CERTIFICATION OF THE PRINCIPAL EXECUTIVE OFFICER PURSUANT TO RULE 13a-14(b) OF
THE SECURITIES EXCHANGE ACT OF 1934 AND 18 U.S.C. SECTION 1350

     In connection with the Quarterly Report on Form 10-Q of Penwest Pharmaceuticals Co. (the “Company”) for the period ended June 30, 2008 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), the undersigned, Jennifer L. Good, 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.

Date: August 6, 2008	/s/ Jennifer L. Good
	Jennifer L. Good
	President and Chief Executive Officer