W I T N E S S E T H:

Exhibit 4.1

EXECUTION COPY

     FIFTH SUPPLEMENTAL INDENTURE, dated as of February 9, 2007, by and among EOP Operating Limited Partnership, a Delaware limited partnership (the “Issuer”), Equity Office Properties Trust, a Maryland real estate investment trust (“EOPT”), Blackhawk Acquisition Trust, a Maryland real estate investment trust (the “Initial Successor”), Blackhawk Parent LLC, a Delaware limited liability company (the “Successor”), U.S. Bank National Association, as trustee (the “Trustee”), and, solely with respect to the Internotes (as defined below), BNY Midwest Trust Company, as trustee (the “Additional Trustee”).

W I T N E S S E T H:

     WHEREAS, the Issuer, EOPT and the Trustee have heretofore executed and delivered the Indenture dated as of August 29, 2000 (as amended and supplemented to the date hereof, the “Indenture”), the Issuer, EOPT and the Additional Trustee have heretofore executed and delivered the New Trustee Appointment Agreement dated as of June 10, 2004, and the Issuer has issued pursuant to the Indenture, among other securities, the following securities: 7.75% Notes due 2007 (the “7.75% Notes”), Floating Rate Notes due 2010 (the “2010 Floating Rate Notes”), 4.65% Notes due 2010 (the “4.65% Notes”), 7.00% Notes due 2011 (the “7.00% Notes”), 6.75% Notes due 2012 (the “6.75% Notes”), 5.875% Notes due 2013 (the “5.875% Notes”), 4.75% Notes due 2014 (the “4.75% Notes”), Floating Rate Notes due 2014 (the “2014 Floating Rate Notes”), 7.875% Notes due 2031 (the “7.875% Notes”), the 4.00% Exchangeable Senior Notes due 2026 (the “Exchangeable Notes”) and the Internotes listed on Schedule 1 hereto (the “Internotes” and together with the 7.75% Notes, the 2010 Floating Rate Notes, the 4.65% Notes, the 7.00% Notes, the 6.75% Notes, the 5.875% Notes, the 4.75% Notes, the 2014 Floating Rate Notes, the 7.875% Notes and the Exchangeable Notes, collectively, the “Notes”);

     WHEREAS, EOPT, the Issuer, the Initial Successor, the Successor and Blackhawk Acquisition, L.P., a Delaware limited partnership, have entered into an Agreement and Plan of Merger dated as of November 19, 2006, as amended to the date hereof (the “Merger Agreement”), which contemplates, among other things, the execution and filing on the date hereof of Articles of Merger (the “Articles of Merger”) with the State Department of Assessments and Taxation of Maryland (the “SDAT”) providing for the merger of EOPT with and into the Initial Successor (the “Merger”), with the Initial Successor continuing its existence under Maryland law;

     WHEREAS, upon the acceptance for record of the Articles of Merger by the SDAT or at such other time thereafter as is provided therein (the “Merger Effective Time”), the Merger will be effective in accordance with the terms of the Articles of Merger and applicable law;

     WHEREAS, in connection with the liquidation of the Initial Successor contemplated by the Merger Agreement, the Initial Successor and the Successor will execute and deliver an Assignment and Assumption Agreement dated the date hereof (the “Assignment and Assumption Agreement”) providing for the assignment to the Successor by the Initial Successor of substantially all of its assets and the assumption by the Successor of all of the liabilities of the Initial Successor;