EXHIBIT 4.1

SUPPLEMENTAL INDENTURE NO. 18

by and between

HRPT PROPERTIES TRUST

and

U.S. BANK NATIONAL ASSOCIATION

as of September 18, 2007

SUPPLEMENTAL TO THE INDENTURE DATED AS OF JULY 9, 1997

HRPT PROPERTIES TRUST

6.65% Senior Notes due 2018

This SUPPLEMENTAL INDENTURE NO. 18 (this “Supplemental Indenture”) made and entered into as of September 18, 2007 between HRPT PROPERTIES TRUST, a Maryland real estate investment trust (the “Company”), and U.S. BANK NATIONAL ASSOCIATION, a national banking association, as trustee (the “Trustee”),

WITNESSETH THAT:

WHEREAS, the Company and the Trustee are parties to an Indenture, dated as of July 9, 1997 (the “Indenture”), relating to the Company’s issuance, from time to time, of various series of debt securities;

WHEREAS, the Company has determined to issue debt securities known as its 6.65% Senior Notes due 2018; and

WHEREAS, the Indenture provides that certain terms and conditions for each series of debt securities issued by the Company thereunder may be set forth in an indenture supplemental to the Indenture;

NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

ARTICLE 1

DEFINED TERMS

Section 1.1              The following definitions supplement, and, to the extent inconsistent with, replace the definitions in Section 101 of the Indenture:

“Acquired Debt” means Debt of a Person or entity (i) existing at the time such Person or entity becomes a Subsidiary or (ii) assumed in connection with the acquisition of assets from such Person or entity, in each case, other than Debt incurred in connection with, or in contemplation of, such Person or entity becoming a Subsidiary or such acquisition.  Acquired Debt shall be deemed to be incurred on the date of the related acquisition of assets from any Person or entity or the date the acquired Person or entity becomes a Subsidiary.

“Annual Debt Service” as of any date means the maximum amount which is expensed in any 12-month period for interest on Debt of the Company and its Subsidiaries.

“Business Day” means any day other than a Saturday or Sunday or a day on which banking institutions in the City of New York or in the city in which the Corporate Trust Office of the Trustee is located, are required or authorized to close.

“Capital Stock” means, with respect to any Person, any capital stock (including preferred stock), shares, interests, participation or other ownership interests (however designated) of such Person and any rights (other than debt securities convertible into or exchangeable for capital stock), warrants or options to purchase any thereof.

“Consolidated Income Available for Debt Service” for any period means Earnings from Operations of the Company and its Subsidiaries plus amounts which have been deducted, and

minus amounts which have been added, for the following (without duplication): (i) interest on Debt of the Company and its Subsidiaries, (ii) provision for taxes of the Company and its Subsidiaries based on income, (iii) amortization of debt discount and deferred financing costs, (iv) provisions for gains and losses on properties and property depreciation and amortization, (v) the effect of any noncash charge resulting from a change in accounting principles in determining Earnings from Operations for such period and (vi) amortization of deferred charges.

“Corporate Trust Office” means the corporate trust office of the Trustee which it designates as the office at which the agreement in question will be administered (which it may change by notice from time to time), presently located at One Federal Street, 3rd Floor, Boston, Massachusetts 02110.

“Debt” of the Company or any Subsidiary means, without duplication, any indebtedness of the Company or any Subsidiary, whether or not contingent, in respect of (i) borrowed money or evidenced by bonds, notes, debentures or similar instruments, (ii) indebtedness for borrowed money secured by any Encumbrance existing on property owned by the Company or any Subsidiary, to the extent of the lesser of (x) the amount of indebtedness so secured and (y) the fair market value of the property subject to such Encumbrance, (iii) the reimbursement obligations, contingent or otherwise, in connection with any letters of credit actually issued (other than letters of credit issued to provide credit enhancement or support with respect to other indebtedness of the Company or any Subsidiary otherwise reflected as Debt hereunder) or amounts representing the balance deferred and unpaid of the purchase price of any property or services, except any such balance that constitutes an accrued expense or trade payable, or all conditional sale obligations or obligations under any title retention agreement, (iv) the principal amount of all obligations of the Company or any Subsidiary with respect to redemption, repayment or other repurchase of any Disqualified Stock, or (v) any lease of property by the Company or any Subsidiary as lessee which is reflected on the Company’s consolidated balance sheet as a capitalized lease in accordance with GAAP, to the extent, in the case of items of indebtedness under (i) through (iii) above, that any such items (other than letters of credit) would appear as a liability on the Company’s consolidated balance sheet in accordance with GAAP, and also includes, to the extent not otherwise included, any obligation by the Company or any Subsidiary to be liable for, or to pay, as obligor, guarantor or otherwise (other than for purposes of collection in the ordinary course of business), Debt of another Person (other than the Company or any Subsidiary) (it being understood that Debt shall be deemed to be incurred by the Company or any Subsidiary whenever the Company or such Subsidiary shall create, assume, guarantee or otherwise become liable in respect thereof).

“Disqualified Stock” means, with respect to any Person, any Capital Stock of such Person which by the terms of such Capital Stock (or by the terms of any security into which it is convertible or for which it is exchangeable or exercisable), upon the happening of any event or otherwise (i) matures or is mandatorily redeemable, pursuant to a sinking fund obligation or otherwise (other than Capital Stock which is redeemable solely in exchange for common stock or shares), (ii) is convertible into or exchangeable or exercisable for Debt or Disqualified Stock, or (iii) is redeemable at the option of the Holder thereof, in whole or in part (other than Capital Stock which is redeemable solely in exchange for common stock or shares), in each case on or prior to the stated maturity of the Notes.

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“Earnings from Operations” for any period means net earnings excluding gains and losses on sales of investments, extraordinary items, gains and losses on early extinguishment of debt and property valuation losses, as reflected in the financial statements of the Company and its Subsidiaries for such period, determined on a consolidated basis in accordance with GAAP.

“Encumbrance” means any mortgage, lien, charge, pledge or security interest of any kind.

“Make-Whole Amount” means, in connection with any optional redemption or accelerated payment of any Notes prior to July 15, 2017, the excess, if any, of (i) the aggregate present value as of the date of such redemption or accelerated payment of each dollar of principal being redeemed or paid and the amount of interest (exclusive of interest accrued to the date of redemption or accelerated payment) that would have been payable in respect of such dollar if such redemption or accelerated payment had been made on July 15, 2017, determined by discounting, on a semiannual basis, such principal and interest at the Reinvestment Rate (determined on the third Business Day preceding the date such notice of redemption is given or declaration of acceleration is made) from the respective dates on which such principal and interest would have been payable if such redemption or accelerated payment had been made on July 15, 2017, over (ii) the aggregate principal amount of the Notes being redeemed or paid.  In the case of any redemption or accelerated payment of notes on or after July 15, 2017, the Make-Whole Amount means zero.  For purposes of this Supplemental Indenture and the Notes, references in the Indenture to the payment of the principal (and premium, if any) and interest on the Notes shall be deemed to include the payment of the Make-Whole Amount, if any, due upon redemption with respect to the Notes.  The Make-Whole Amount shall be calculated by the Company and set forth in an Officer’s Certificate delivered to the Trustee, and the Trustee shall be entitled to rely on said Officer’s Certificate.

“Notes” means the Company’s 6.65% Senior Notes due 2018, issued under this Supplemental Indenture and the Indenture, as amended or supplemented from time to time.

“Reinvestment Rate” means a rate per annum equal to the sum of 0.40% (forty one-hundredths of one percent) plus the yield on treasury securities at constant maturity under the heading “Week Ending” published in the Statistical Release under the caption “Treasury Constant Maturities” for the maturity (rounded to the nearest month) corresponding to the remaining life to maturity (which, in the case of maturities corresponding to the principal and interest due on the notes at their maturity, shall be deemed to be July 15, 2017), as of the payment date of the principal being redeemed or paid.  If no maturity exactly corresponds to such maturity, yields for the two published maturities most closely corresponding to such maturity shall be calculated pursuant to the immediately preceding sentence and the Reinvestment Rate shall be interpolated or extrapolated from such yields on a straight-line basis, rounding in each of such relevant periods to the nearest month.  For purposes of calculating the Reinvestment Rate, the most recent Statistical Release published prior to the date of determination of the Make-Whole Amount shall be used.

“Secured Debt” means Debt secured by any mortgage, lien, charge, pledge or security interest of any kind.

“Statistical Release” means the statistical release designated “H.15(519)” or any successor publication which is published weekly by the Federal Reserve System and which

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establishes yields on actively traded United States government securities adjusted to constant maturities or, if such statistical release is not published at the time of any determination under this Supplemental Indenture, then any publicly available source of similar market data which shall be designated by the Company.

“Subsidiary” means any corporation or other entity of which a majority of (i) the voting power of the voting equity securities or (ii) the outstanding equity interests are owned, directly or indirectly, by the Company or one or more other Subsidiaries of the Company.  For the purposes of this definition, “voting equity securities” means equity securities having voting power for the election of directors, whether at all times or only so long as no senior class of security has such voting power by reason of any contingency.

“Total Assets” as of any date means the sum of (i) the Undepreciated Real Estate Assets and (ii) all other assets of the Company and its Subsidiaries determined in accordance with GAAP (but excluding accounts receivable and intangibles).

“Total Unencumbered Assets” means the sum of (i) those Undepreciated Real Estate Assets not subject to an Encumbrance for borrowed money and (ii) all other assets of the Company and its Subsidiaries not subject to an Encumbrance for borrowed money determined in accordance with GAAP (but excluding accounts receivable and intangibles).

“Undepreciated Real Estate Assets” as of any date means the cost (original cost plus capital improvements) of real estate assets of the Company and its Subsidiaries on such date, before depreciation and amortization, determined on a consolidated basis in accordance with GAAP.

“Unsecured Debt” means Debt which is not secured by any of the properties of the Company or any Subsidiary.

ARTICLE 2

TERMS OF THE NOTES

Section 2.1              Pursuant to Section 301 of the Indenture, the Notes shall have the following terms and conditions:

(a)           Title; Aggregate Principal Amount; Form of Notes .  The Notes shall be Registered Securities under the Indenture and shall be known as the Company’s “6.65% Senior Notes due 2018.”  The Notes will be limited to an aggregate principal amount of $250,000,000, subject to the right of the Company to reopen such series for issuances of additional securities of such series and except as provided in this Section or in Section 306 of the Indenture.  The Notes (together with the Trustee’s certificate of authentication) shall be substantially in the form of Exhibit A hereto, which is hereby incorporated in and made a part of this Supplemental Indenture.

The Notes will be issued in the form of one or more registered global securities without coupons (“Global Notes”) that will be deposited with, or on behalf of, The Depository Trust Company (“DTC”), and registered in the name of DTC’s nominee, Cede & Co.  Except under the circumstance described below, the Notes will not be issuable in definitive form.  Unless and until it is exchanged in whole or in part for the individual Notes represented thereby, a Global Note

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may not be transferred except as a whole by DTC to a nominee of DTC or by a nominee of DTC to DTC or another nominee of DTC or by DTC or any nominee of DTC to a successor depositary or any nominee of such successor.

So long as DTC or its nominee is the registered owner of a Global Note, DTC or such nominee, as the case may be, will be considered the sole owner or holder of the Notes represented by such Global Note for all purposes under this Supplemental Indenture.  Except as described below, owners of beneficial interest in Notes evidenced by a Global Note will not be entitled to have any of the individual Notes represented by such Global Note registered in their names, will not receive or be entitled to receive physical delivery of any such Notes in definitive form and will not be considered the owners or holders thereof under the Indenture or this Supplemental Indenture.

If DTC is at any time unwilling, unable or ineligible to continue as depositary and a successor depositary is not appointed by the Company within 90 days, the Company will issue individual Notes in exchange for the Global Note or Global Notes representing such Notes.  In addition, the Company may at any time and in its sole discretion, subject to certain limitations set forth in the Indenture, determine not to have any of such Notes represented by one or more Global Notes and, in such event, will issue individual Notes in exchange for the Global Note or Global Notes representing the Notes.  Individual Notes so issued will be issued in denominations of $1,000 and integral multiples thereof.

(b)           Interest and Interest Rate .  The Notes will bear interest at a rate of 6.65%  per annum, from September 18, 2007 (or, in the case of Notes issued upon any reopening of this series of Notes, from the date designated by the Company in connection with such reopening) or from the immediately preceding Interest Payment Date to which interest has been paid or duly provided for, payable semiannually in arrears on each January 15 and July 15, commencing January 15, 2008 (each of which shall be an “Interest Payment Date”), to the Persons in whose names the Notes are registered in the Security Register at the close of business on the day falling 14 calendar days (whether or not a Business Day) nex