This article explores the legal principles that govern the interpretation of “secondary instruments” in international law. A “secondary instrument” under international law is, for the purposes of this article, a written document adopted by a body empowered by a treaty to take action with respect to the treaty, but which is not itself a treaty. Such instruments find increasing application in international law. The article specifically examines the interpretation of secondary instruments arising in five settings in international practice: the United Nations Security Council, the International Maritime Organization, the International Seabed Authority, the International Whaling Commission, and conferences/meetings of the parties under multilateral treaties. This selection of practice will serve to illustrate principles of interpretation across a range of international institutional settings for the purpose of determining the rights and obligations of state-parties to a treaty regime.

The United Nations Convention on the Law of the Sea (UNCLOS), referred to as the “constitution of the seas and oceans”, established the legal regime of the seas and oceans. Under the United Nations Convention on the Law of the Sea, the International Maritime Organization (IMO) has a mandate as a global legislative entity to further regulate maritime issues on the basis of many of its provisions. IMO is explicitly mentioned in only one of the articles of UNCLOS, while several provisions of the Convention on the Law of the Sea refer to the ‘competent international organization’ in connection with the adoption of international rules and standards for shipping in matters relating to maritime safety and the prevention, reduction and control of marine pollution by ships. Bearing in mind the global mandate of the Organization as a specialized agency within the United Nations system established by the Convention on the International Maritime Organization, the expression ‘competent international organization’, when used in the singular in UNCLOS, applies to IMO. From 1973 to 1982, the International Maritime Organization actively participated in the work of the Third United Nations Conference on the Law of the Sea to ensure that the development of IMO instruments was in line with the fundamental principles of UNCLOS. By including in several IMO conventions provisions that clearly state that these conventions do not infringe the codification and development of the law of the sea in UNCLOS, many interpretation doubts have been minimized.