Web versions of multilateral treaties to which the United States may or may not be a party are relatively easy to find.
The following resources on the University of Iowa Law Library Electronic Resources page may be helpful when researching multilateral treaties:
Provides full text access to published and unpublished treaties drawn from: the Statutes at Large; United States Treaties and Other International Agreements (UST); Treaties and International Act Series (TIAS); KAV Agreements; Indian Affairs, Laws, and Treaties; International Legal Materials; and more. This collection also contains related books and texts including the American Diplomatic Code and the Department of State Bulletin.
The following Web resources offer full text of many multilateral treaties:
Among the materials included in this collection are: Status of Multilateral Treaties Deposited with the Secretary-General, which provides information on the status of over 500 major multilateral instruments; United Nations Treaty Series, containing treaties and agreements registered or filed between December 14, 1946 - January, 2005; League of Nations Treaty Series, containing the treaties published by the League of Nations between July 5, 1920 - October 3, 1944.