The EU is often described as a sui generis international organization. While this is an apt description in some ways, it is far from self-evident that the special features that the EU exhibits are or should be part of the explanation of why and how it is bound by customary international law. This chapter seeks to show that, in the eye of public international law, the EU is not yet in a category of its own which would justify or otherwise affect the applicability of custom to it. It does so by investigating whether a special category of ‘regional economic integration organizations’ (REIOs) has emerged to which international law grants ‘preferential’ treatment, with a focus on three rules that were unsuccessfully proposed to cater for the specificities of the EU in codification projects carried out by the UN International Law Commission. The chapter also considers doctrinal and normative reasons why calls to distinguish the EU from other international organizations under customary international law have fallen on deaf ears, while recognizing the role that the EU has played in widening the horizons of the law of international organizations.