Exhibit 5.1

July 14, 2006		Mayer, Brown, Rowe & Maw LLP
		71 South Wacker Drive
		Chicago, Illinois 60606-4637
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Archstone-Smith Operating Trust		www.mayerbrownrowe.com
9200 East Panorama Circle, Suite 400
Englewood, Colorado 80112

Re: Registration Statement on Form S-3

Ladies and Gentlemen:

     We have acted as special counsel to Archstone-Smith Operating Trust, a Maryland real estate investment trust (“Archstone”), in connection with (i) an offering pursuant to Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”), of Archstone’s Debt Securities, and the corporate proceedings taken in connection therewith and (ii) the issuance of $500 million aggregate principal amount of 4.00% Exchangeable Senior Notes due 2036 and (the “Notes”). The Notes are to be issued (1) under the Indenture dated as of February 1, 1994, between Archstone and Morgan Guaranty Trust Company of New York, as trustee (“Morgan”), as supplemented by the First Supplemental Indenture dated as of February 2, 1994, between Archstone, Morgan and U.S. Bank National Association (as successor in interest to State Street Bank and Trust Company), as successor trustee (the “Trustee”), as further supplemented by the Second Supplemental Indenture dated as of August 2, 2004, between Archstone and the Trustee, and as further supplemented by the Third Supplemental Indenture dated as of July 14, 2006, between Archstone and the Trustee (the “Third Supplemental Indenture”) (collectively, the “Indenture”), and (2) subject to an Underwriting Agreement, dated July 11, 2006 (the “Underwriting Agreement”), among Archstone, Archstone-Smith Trust, Citigroup Global Markets Inc. and Morgan Stanley & Co. Incorporated, as representatives of the several underwriters named in the Underwriting Agreement.

     We have also participated in the preparation and filing with the Securities and Exchange Commission under the Securities Act of a Registration Statement on Form S-3, as amended (File No. 333-133285) (the “Registration Statement”), relating to the debt securities of which the Notes are a part. In rendering our opinions set forth below, we have examined originals or copies identified to our satisfaction of (i) the Underwriting Agreement; (ii) the Registration Statement; (iii) the prospectus as supplemented relating to the Notes; (iv) the Indenture; (v) the form of the Notes; and (vi) the Third Supplemental Indenture. The Notes are registered on Registration Statement No. 333-133285. In addition, we have examined and relied upon other documents, certificates, corporate records, opinions and instruments, obtained from Archstone or

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MAYER, BROWN, ROWE & MAW LLP

July 14, 2006
Page 2

other sources believed by us to be reliable, as we have deemed necessary or appropriate for the purpose of this opinion.

     As special counsel to Archstone, we have examined originals or copies certified or otherwise identified to our satisfaction of the Declaration of Trust, Archstone’s Amended and Restated Bylaws, as amended, resolutions of the sole Trustee of Archstone and such Archstone records, certificates and other documents and such questions of law as we considered necessary or appropriate for the purpose of this opinion. As to certain facts material to our opinion, we have relied, to the extent we deem such reliance proper, upon certificates of public officials and officers of Archstone. In rendering this opinion, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals and the conformity to authentic original documents of all documents submitted to us as copies.

     Based upon and subject to the foregoing and to the assumptions, conditions and limitations set forth herein, and assuming that the Notes are issued in accordance with the terms of the Underwriting Agreement, we are of the opinion that the Notes constitute valid and legally binding obligations of Archstone entitled to the benefits of the Indenture, except that (a) the enforceability thereof may be subject to (i) bankruptcy, insolvency, reorganization, moratorium, fraudulent conveyance or other similar laws now or hereafter in effect relating to or affecting creditors’ rights or remedies generally and (ii) general principles of equity and to the discretion of the court before which any proceedings therefor may be brought (regardless of whether enforcement is sought in a proceeding at law or in equity) and (b) the enforceability of provisions imposing liquidated damages, penalties or an increase in interest rate upon the occurrence of certain events may be limited in certain circumstances ((a) and (b), collectively, the “Enforceability Exceptions”).

     We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to being named in the related prospectus and any related prospectus supplement under the caption “Legal Matters” with respect to the matters stated therein.

     We are admitted to practice law in the State of Illinois, and we express no opinion as to matters under or involving any laws other than the laws of the State of Illinois, the laws of the State of New York, the federal laws of the United States of America and the laws of the State of Maryland.

Sincerely,

/s/ MAYER, BROWN, ROWE & MAW LLP