Bilateral relations between states which are parties to a multilateral treaty may not be governed by that treaty. This may depend on an agreement between the two states concerned, which could subject their bilateral relations to a different regime that is considered to suit their specific needs better. The exclusion of bilateral relations under the multilateral treaty may also be the consequence of a unilateral expression of the will of one of these states, for instance in view of its non-recognition of the other state. The present article seeks to examine the conditions under which bilateral relations may be excluded on the basis of the unilateral determination of one of the states concerned.

The law of unilateral acts and the law of treaties are generally considered to be two distinct areas of international law. While the former governs the effect of unilateral undertakings by States, the latter governs, inter alia, the formation, interpretation, termination, and suspension of treaties. In 2011, the International Law Commission (ILC) concluded a two-decade study on reservations. One of its most remarkable insights is the argument that reservations ought to be considered ‘unilateral acts’. Thus, certain rules pertaining the law of unilateral acts ought to apply to reservations. This article critically examines the ILC's novel proposal as well as its attempt to provide answers to contemporary questions on reservations through conceptualizing reservations as unilateral acts.

Originally it was suggested that I write about ‘rusty’ treaties – by which was meant ‘old’ treaties that have not been terminated – and their continued relevance in view of the principle of rebus sic stantibus, now enshrined in Article 62 of the Vienna Convention on the Law of Treaties 1969. I did not relish that. There would be problems of definition: when can a treaty be said to be ‘old’? Also, I remain sceptical about the usefulness of the fundamental change principle, being of the view that it is of little real importance. Although invoked many times, so far no international tribunal has applied it. The editors therefore kindly agreed that I could instead write about treaties of a universal character, in that they are open to universal participation, but which are ‘limping’.