Exhibit 32.1

Certification Pursuant to 18 U.S.C. Sec. 1350

(Section 906 of the Sarbanes — Oxley Act of 2002)

In connection with the filing by CommonWealth REIT (the “Company”) of the Quarterly Report on Form 10-Q for the period ended March 31, 2011 (the “Report”), each of the undersigned hereby certifies, to the best of his knowledge:

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/ Barry M. Portnoy	/s/ Adam D. Portnoy
Barry M. Portnoy	Adam D. Portnoy
Managing Trustee	President and Managing Trustee
/s/ John C. Popeo
John C. Popeo
Treasurer and Chief Financial Officer
Date: May 6, 2011