Diamonds occur in modern alluvial deposits at several localities in Myanmar, Thailand, and Sumatra; the deposits do not contain typical diamond indicator minerals, and no obvious primary sources have been found in the surrounding terrane, which is characterized by Phanerozoic tectonic and magmatic activity. Detailed studies of diamonds from Theindaw and Momeik in Myanmar and Phuket in Thailand have been undertaken to clarify their origin. Syngenetic mineral inclusions in the diamonds are largely of the peridotitic paragenesis, with a smaller eclogitic component. Carbon and nitrogen isotope compositions are typical of kimberlitic and lamproitic diamond suites worldwide. Nitrogen aggregation states indicate a long residence and/or significant deformation at mantle temperatures; many stones show plastic deformation features. The rounded and polished surfaces of most of the diamonds reflect resorption in a corrosive magma. These features do not support an exotic or unusual origin for the diamonds, for example, by subduction-exhumation. Extensive abrasion and abundant brown radiation damage spots suggest long surface transport. Their distribution within the Sibumasu terrane, and their close association with Carbo-Permian glacial-marine sediments, suggest that these diamonds were derived from primary sources in northwestern Australia or within the terrane itself, prior to the Early Permian separation of the Sibumasu terrane from the Gondwanaland margin. The isotopic data and the dominantly peridotitic nature of syngenetic inclusions rule out the Argyle lamproite as a possible source and also distinguish the Myanmar- Thailand diamonds from morphologically similar stones from eastern Australia.