The Court of Justice of the European Union has on numerous occasions employed the provisions of the Vienna Convention on the Law of Treaties, to the extent that they represent principles of customary international law, in its judicial reasoning. At first glance, the Court’s use of the Vienna rules demonstrate fidelity towards international law; it can be seen as contributing to the ‘strict observance and the development of international law’. Upon closer examination, however, one finds that the Court applies these rules in a fashion that often deviates from the way in which other courts and bodies have applied the same principles. This article examines how the Court has used international treaty law, arguing that the Court often employs a novel, ‘European’ approach to certain principles. While the Court is free to apply treaty law in a manner it believes to be appropriate, the extent of this divergence risks undermining the integrity and uniform application of the Vienna rules.