Article 41 Vienna Convention on the Law of Treaties
Agreements to modify multilateral treaties between certain of the parties only
1. Two or more of the parties to a multilateral treaty may conclude an agreement to modify the treaty as between themselves alone if:
(a) the possibility of such a modification is provided for by the treaty; or
(b) the modification in question is not prohibited by the treaty and:
(i) does not affect the enjoyment by the other parties of their rights under the treaty or the performance of their obligations;
(ii) does not relate to a provision, derogation from which is incompatible with the effective execution of the object and purpose of the treaty as a whole.
2. Unless in a case falling under paragraph 1 (a) the treaty otherwise provides, the parties in question shall notify the other parties of their intention to conclude the agreement and of the modification to the treaty for which it provides.
See also: Article 40 Vienna Convention on the Law of Treaties
