Indonesia's cooperation in maritime security initiatives is vitally important because half of the world's trading goods and oil pass through Indonesian waters including the Straits of Malacca, the Strait of Sunda, and the Strait of Lombok. However, Indonesia has a mixed record of support for various initiatives led by the United States, joining certain arrangements but opting out of others. This article explains why by carrying out a comparative analysis of four maritime cooperation arrangements, using government documents and elite interviews in Southeast Asia. This article argues that Indonesia's decision to cooperate is formed by the calculation of absolute gains. Indonesia chose to cooperate whenever it saw that the benefits of cooperation exceeded the costs.