The exceptio non adimpleti contractus in public international law

The position of the exceptio non adimpleti contractus in public international law as a response to the breach of a treaty is surrounded by uncertainty. This article offers an analytical examination of: i) the notion of the exceptio and its relationship to other responses to the breach of a treaty; ii) the question of its conditions of application; and iii) its character as a rule of international law. It argues that the exceptio is distinct both from responses pursuant to Article 60 of the Vienna Convention on the Law of Treaties (VCLT) and from countermeasures; however, there is uncertainty with regard to its conditions of application. The exceptio can be considered as a general principle of law and, as such, can fill gaps in the regulation of responses to the breach of a treaty. However, it remains unclear whether the exceptioconstitutes a rule of customary international law.