Vienna Convention on Treaties.
signed by US in 1970.
From US Dept. of State website:
Is the United States a party to the Vienna Convention on the Law of Treaties?
No. The United States signed the treaty on April 24, 1970. The U.S. Senate has not given its advice and consent to the treaty. The United States considers many of the provisions of the Vienna Convention on the Law of Treaties to constitute customary international law on the law of treaties.
Found here:
http://www.state.gov/s/l/treaty/faqs/70139.htm