The interaction between domestic law and international law is a topic of perennial interest for international lawyers. Domestic law has long been recognised as a source of international law, an inspiration for legal developments, or the benchmark against which a legal system is to be assessed. More often than not, it is simply treated as mere fact, indicative of the legality of a state’s actions. Academic commentary invariably re-traces these well-trodden paths, leaving one with the impression that the interaction between domestic and international law has been thoroughly mapped, unworthy of further enquiry. However, a different – and surprisingly pervasive – nexus between the two spheres has been largely overlooked: the use of domestic law in the interpretation of international law. The present thesis fills this gap in the literature.

This thesis aims to answer two questions: first, is domestic law used in the interpretation of international law by international courts and tribunals; and, second, is it permissible for courts and tribunals to use domestic law in this way? Despite their deceptively simple appearance, these questions raise issues that go to the very heart of interpretation itself. On what basis, for example, can we say that an interpretation is permissible in a certain context? Do the provisions of the Vienna Convention on the Law of Treaties (VCLT) constrain the choice of interpretive methods, and do they provide a framework within which interpretation can – or must – be evaluated? Are there other frameworks for evaluation that more accurately describe when and why a particular interpretation will be accepted by its addressees and others within the legal regime? Only after adequately addressing these questions will it be possible to properly examine the use of domestic law by international courts and tribunals.

This thesis is hence not a doctrinal exegesis of the place of domestic law within the Vienna Convention articles, nor does it provide an exhaustive typology of the uses of domestic law by international courts. Rather, it examines the use of domestic law in order to challenge the conventional views regarding the centrality of the Vienna Convention provisions to interpretation, whilst also providing a fresh perspective on the interaction between international and domestic law. It is only when we break free from the ‘conceptual straightjacket’ of Article 31 that we can truly understand whether domestic law has a place in the interpretation of international law.