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.