This article first examines the entire range of agency relationships between an international organisation, acting as a principal, and one or more states, acting as its agent(s). It is here argued that an agency relationship between an international organisation and one or more state(s) can be established by the constitutive treaty of the organisation, or ad hoc. In particular, an ad hoc agency relationship results from a state placing one of its organs at the disposal of an international organisation and under its effective control, with the purpose of carrying out functions of that organisation.

The article then examines the consequences for the responsibility of the organisation and the relevant state(s) of an agency relationship between an international organisation acting as a principal and one or more states acting as its agent(s). It is demonstrated that an international organisation may be responsible for damage caused by the conduct of the state. Furthermore, it is argued that the state itself may bear responsibility for having established or for not having terminated the agency relationship if it commits wrongful conduct on behalf of the international organisation.

The article discusses the question of the international responsibility of the European Community against the background of the International Law Commission's (ILC) recent work on the responsibility of international organizations generally. It describes the role of the EC (and EU) in international relations and highlights its special legal features in the light of plentiful practical examples. The various practical issues drawn from the EC's voluminous international treaty practice are grouped into what the paper calls “horizontal” and “vertical” issues, illustrating the different aspects related to the division of competence between the organization and its Member States and the operating features of the EC legal system reflecting its nature as “executive federalism”. The underlying theme of the paper recognizes that there are important “public morals” aspects calling for clear rules on the responsibility of international organizations, but there are also practical realities related to the diversity of international organizations pointing to the direction of special rules for instance in the case of the EC. On the basis of a broad review of the EC practice, the paper offers some critical comments on the ILC work suggesting that the traditional notion of an international organization does not resonate well with the realities of the European Community. In particular it argues that the ILC has overlooked the notions such as the Regional Economic Integration Organization (REIO), which has emerged in multilateral treaty practice during recent years, and failed to consider broader international law implications that such practice entails, including for purposes of international responsibility.