Article 32 of the 1969 Vienna Convention on the Law of Treaties states that one can consult “supplementary means of interpretation,” including “the preparatory work of the treaty” when the ordinary meaning of a treaty term is “ambiguous” or leads to an “unreasonable” result. Vienna Convention on the Law of Treaties art. 32, May 23, 1969, 1155 U.N.T.S. 331. The Medellin Court characterized treaty negotiating and drafting history and the "postratification understanding of signatory nations" as “aids" to treaty interpretation. Medellin v. Texas, 128 S.Ct. 1346, 1357 (2008), quoting Zicherman v. Korean Air Lines Co., 516 U.S. 217, 226 (1996). Thus, the drafting history of a treaty, which people sometimes refer to by the French term “travaux préparatoires,” can be an important source.
TIP: when searching catalogs like SARA, combine words from the title of the treaty with:
history or travaux préparatoires or commentary
The United Nations provides a searchable database of official records from eight UN-sponsored treaty conferences in: United Nations Diplomatic Conferences.
Jonathan Pratter links to many websites that provide treaty drafting history in: UPDATE: À la Recherche des Travaux Préparatoires: An Approach to Researching the Drafting History of International Agreements.