Article 40 Vienna Convention on the Law of Treaties
Amendment of multilateral treaties
1. Unless the treaty otherwise provides, the amendment of multilateral treaties shall be governed by the following paragraphs.
2. Any proposal to amend a multilateral treaty as between all the parties must be notified to all the contracting States, each one of which shall have the right to take part in:
(a) the decision as to the action to be taken in regard to such proposal;
(b) the negotiation and conclusion of any agreement for the amendment of the treaty.
3. Every State entitled to become a party to the treaty shall also be entitled to become a party to
the treaty as amended.
4. The amending agreement does not bind any State already a party to the treaty which does not become a party to the amending agreement; article 30, paragraph 4 (b), applies in relation to such State.
5. Any State which becomes a party to the treaty after the entry into force of the amending agreement shall, failing an expression of a different intention by that State:
(a) be considered as a party to the treaty as amended; and
(b) be considered as a party to the unamended treaty in relation to any party to the treaty not bound
by the amending agreement.
See also: Article 41 Vienna Convention on the Law of Treaties