The Vienna Convention on the Law of Treaties between States and International Organizations or Between International Organizations (VCLTIO) is an extension of the Vienna Convention on the Law of Treaties which deals with treaties between States. It was developed by the International Law Commission and opened for signature on March 21, 1986.

Article 85 of the Convention provides that it enters into force after the ratification by 35 states (international organizations may ratify, but their ratification does not count towards the number required for entry into force). On 13 August 2011, the UN Treaty Database listed 41 parties to the Convention, but only 29 states. As a result the Convention is not yet in force.^[1]