b) The explicit amendment
For bilateral treaties the rule incorporated in Article 39 according to which a treaty can be amended by agreement between the parties seems sufficient. As for multilateral treaties, one can envisage two types of situation:
- either two or more States make a proposal for an amendment
designed to enter into force for all States parties
- or they intend to modify the treaty only between themselves.
In both cases, in view of the
good faith principle, the acting parties have to notify the others of their intention to conclude the agreement and of the envisaged modification to the treaty (Article 40 (2) and
42 (2)).
According to the principle that agreements do not bind parties which are not privy to it (Article 34, repeated in Article 30 (4)(b) -
pacta
tertiis nec nocent nec prosunt), a modification agreed upon between certain States cannot bind other States which have not approved the modification or amendment (Article 40 (4)), despite the fact that the latter are parties to the original treaty.
Out of respect for the principles of
free consent and
good faith, an agreement aiming at modifying a multilateral treaty only between certain of its parties, must not infringe
- either upon the rights and obligations of the parties to the original
treaty
- or upon its object and purpose (Article 41(1)(b)(ii) - a vague
concept, which is reiterated in the Articles 18, 19 and 31(1)).
Finally, it is worthwhile noting that the
Vienna Convention distinguishes between amendments which are valid for all parties to a given treaty and modifications which are only relevant for a restricted number of them.
