The Modification of Treaties by Subsequent Practice: The Implications of Practice Going Beyond the Limits of Treaty Interpretation

Abstract

International treaties can be notoriously difficult to amend by formal procedures. They must nevertheless be adapted over time to their changing international environment. Subsequent practice – a well-established tool for treaty interpretation – sometimes diverges from treaty provisions to such an extent that it can no longer be said to constitute an act of interpretation or application, but rather becomes, in effect, one of treaty modification. After examining the parameters of the notion of ‘subsequent practice’ and its application in the process of treaty interpretation in light of Articles 31 and 32 of the 1969 Vienna Convention on the Law of Treaties, the present study focuses on its role as a means of modifying treaty provisions. It demonstrates that the ultimate deletion of the International Law Commission’s draft article on treaty modification by subsequent practice at the Vienna Conference in 1968 did not detract from the validity of the process. The study then explores ways of distinguishing between interpretation and modification, and the importance of doing so for States and international dispute settlement bodies alike. It examines the factors conducive to the process of treaty modification by subsequent practice, its main limitations, and alternative adaptation mechanisms. It is necessary to look at factors such as the age, field, and nature (technical or general, bilateral or multilateral, reciprocal or non-reciprocal) of the treaty or provision in question, and the applicability of special principles. Moreover, one should take into account the presence of ‘in-built’ adaptation mechanisms that could detract from the potential for modification by subsequent practice, such as dispute settlement, evolutionary interpretation, and the doctrine of implied powers. Next, the study explains the treaty modifying potential of subsequent customary law, based on the interplay between customary law and treaties, and its parallels to, and overlap with, the process of modification by subsequent practice. Finally, these results are further illustrated by means of a repertoire of practice – a systematic collection of examples of potential treaty modifications by subsequent practice spanning a wide range of fields and treaty regimes, as assessed within and outside of the case law and the context of international organizations. The study shows that subsequent practice can – under carefully defined conditions, and subject to a high threshold that ensures its strict accordance with the will of the parties – alter, supplement, and terminate treaty provisions or even entire treaty frameworks, as well as previously established modifications by subsequent practice. It can also generate customary law and fuel regime interaction. As a recognised process, dispute settlement bodies have a duty to acknowledge treaty modifications by subsequent practice where these have occurred. The study thus contributes to a deeper understanding of the process, and its continued relevance in the development of international law.