with regard to the amendment of treaties
The rules concerning the conclusion of treaties (Articles 6 to 25) equally apply for the amendment of treaties (Article 39). Because of the pronciple of free consent, the States Parties are free to create specific norms for the amendment of their treaty or treaties. Such an agreement can be entered into
- either implicitely
- or explicitly,
both orally or in written form (the Vienna Convention, however, is only applicable for agreements concluded in written form). This agreement can authorize the amendment of the original treaty as well as it can prohibit or restrict amendments. Nevertheless, agreements concerning the amendment of treaties can also be subject to amendments themselves.
a) The implicit amendment
An amendment is done implicitly when the States Parties conclude successive treaties relating to the same-subject-matter. Unless otherwise provided for in an agreement, the earlier treaty applies only to the extent its provisions are compatible with the later treaty (in conformity with the latin maxim lex posterior derogat legi priori according to which the later treaty prevails over an earlier one if both regulate the same subject-matter, Article 30 (3)).
In view of the provisions concerning e x p l i c i t amendments between certain parties only (Article 41) the implicit amendment must be considered as problematic, since it does not warrant the same transparency in the course of a negotiation:
In particular, parties which want to modify a treaty in accordance with Article 41, para. 2 have to notify the other parties of their intention. This provisions aims at ensuring that negotiations are conducted in good faith with parties being aware of the relevant circumstances.
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.
