Under the terms of the Vienna Convention on the Law of Treaties of 1986, the definition of “treaty” is expanded from the 1969 definition (only between States) to include legally binding agreements between States and international organizations, as well as between international organizations.
The most classic example of a “treaty” between an international organization and a State is a headquarters agreements, which regulates the status and privileges of an international organization in the territory of a host State. Some organizations (including their agencies) such as the United Nations have implantation in several countries (United States, Switzerland are the main ones).
EUCLID has signed two headquarters agreements that are legally in force and in operation. The initial agreement with the Central African Republic (2011) and a new agreement was signed with the Republic of The Gambia in 2013 due to instability and operational challenges in Bangui. In addition, the EUCLID treaty did provide for a temporary headquarters office in Brussels, Belgium (without specific status), and EUCLID maintains a Liaison Office in Washington DC which likewise is an executive office with no special legal status.
Headquarters agreements are submitted to the United Nations Treaty Section for information but are not always published in the UNTS catalog.
Full text of the Headquarters Agreement of 2011
Full text of the Headquarters Agreement of 2013