EXHIBIT 5.1

January 28, 2013

Buckeye Partners, L.P.

One Greenway Plaza Suite 600

Houston, Texas 77046

Ladies and Gentlemen:

We have acted as counsel to Buckeye Partners, L.P., a Delaware limited partnership (the “Partnership”), in connection with (i) the offer and sale by the Partnership of 6,000,000 units representing limited partner interests in the Partnership (the “Offered Units”) pursuant to an Underwriting Agreement, dated as of January 24, 2013 (the “Underwriting Agreement”), by and among Buckeye GP LLC, a Delaware limited liability company (the “General Partner”), the Partnership, and Barclays Capital Inc., Citigroup Global Markets Inc., Morgan Stanley & Co. LLC, Wells Fargo Securities, LLC, Deutsche Bank Securities Inc., J.P. Morgan Securities LLC and UBS Securities LLC as representatives of the several Underwriters listed in Schedule 1 thereof (the “Underwriters”), (ii) the potential offering and sale of up to an additional 900,000 units representing limited partner interests in the Partnership (the “Option Units” and, collectively with the Offered Units, the “Units”) pursuant to the exercise by the Underwriters of an option to purchase the Option Units, and (iii) the filing of the Registration Statement on Form S-3 (Registration No. 333-178097) (the “Registration Statement”) and the prospectus included therein (the “Base Prospectus”) by the Partnership under the Securities Act of 1933, as amended (the “Act”), with the Securities and Exchange Commission (the “SEC”), pursuant to which the offering and sale of the Units are registered. On January 24, 2013, the Partnership filed with the SEC the Base Prospectus and a prospectus supplement dated January 24, 2013 (the “Prospectus Supplement”) pursuant to Rule 424(b) promulgated under the Act.

In connection with the opinion set forth below, we have examined and relied upon the following:

In connection with rendering the opinion set forth below, we have assumed that (i) all information contained in all documents reviewed by us is true and correct; (ii) all signatures on all documents examined by us are genuine; (iii) all documents submitted to us as originals are authentic and all documents submitted to us as copies conform to the originals of those documents.

Based on the foregoing and on such legal considerations as we deem relevant, we are of the opinion that when the Units have been issued and delivered in accordance with the terms of the Underwriting Agreement and upon payment of the consideration therefor provided for therein, such Units will be validly issued and, under the Delaware Revised Uniform Limited Partnership Act (the “Delaware Act”), the purchasers of the Units will have no obligation to make payments for their purchase of such Units (other than the initial purchase price) or contributions to the Partnership solely by reason of their ownership of the Units or their status as limited partners of the Partnership.

With respect to the lack of obligation to make contributions to the Partnership, the Delaware Act provides that a limited partner who receives a distribution after which the liabilities of the Partnership, other than liabilities to partners on account of their partnership interests and liabilities for which the recourse of creditors is limited to specific property of the Partnership, would exceed the fair value of the assets of the Partnership and knew at the time the distribution was in violation of the Delaware Act will be liable to the Partnership for the amount of the distribution for three years from the date of the distribution. Further, the Delaware Act provides that, upon the winding up of the Partnership, if (a) the Partnership distributes its assets other than in the following order: (i) to creditors in satisfaction of their liabilities; (ii) to partners and former partners in satisfaction of liabilities for distributions owed under the Partnership Agreement; (iii) to partners for the return of their contribution; and (iv) to the partners in the proportions in which the partners share in distributions and (b) a limited partner receives a distribution of assets other than the order described in (a) and knew at time the distribution was other than the order described in (a) will be liable to the Partnership for three years from the date of the distribution.

The opinion expressed is limited in all respects to the Delaware Act, the Delaware Limited Liability Company Act, the laws of the State of New York and the federal laws of the United States of America and we are expressing no opinion as to the effect of the laws of any other jurisdiction, domestic or foreign.

We hereby consent to the filing of this opinion as Exhibit 5.1 to the Current Report on Form 8-K of the Partnership dated on or about the date hereof, to the incorporation by reference of this opinion into the Registration Statement and the reference to our firm under the heading “Legal Matters” in the Prospectus Supplement and the Prospectus. In giving such consent, we do not admit that we are within the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the SEC issued thereunder.