Recently, much attention has been paid to the pollution of coastal sea water with various hydrocarbons, which often give rise to a serious damage to the coastal fisheries in Japan. The present rezearch aims to elucidate components of hydrocarbon pollutants in relation to the source and mechanism of formation of so-called tar-balls floating on the surface of sea water. In the preliminary experiments, the hydrocarbon compounds in C-class heavy oil, wasted lubricant, and Iranian, Arabian and Minas crude oil samples were analyzed by gas chromatography (GLC). Then, each oil sample was floated in a test tank and allowed to settle on the surface of sea water for about 4 months, The n-paraffine components in each test sample were analyzed periodically. After standing for 400hr, the hydrocarbon components in the C-class heavy oil, wasted lubricant, Iranian and Arabian crude oil became indistinguishable form one another, while the Minas crude oil was distinctive from other oil samples. The n-paraffine components in tar-balls collected around Hachijo and Okinawa Islands were analyzed by GLC, and the source and mechanism of formation of such types of solid or semi-solid pollutants were discussed.