The composition of cloud condensation nuclei (CCN) was determined by thermal analysis, whereby CCN and aerosol particles were heated to temperatures up to 500¿C. The CCN were observed in the urban atmosphere of Nagoya and in the coastal atmosphere of Mikuni, Japan. Automobile exhaust particles were also analyzed to examine the contribution of anthropogenic organic materials to CCN. The thermal analytical behavior of CCN at Nagoya and Mikuni showed that more than 80% of the total number concentration of CCN evaporated at or below 300¿C and the concentration of CCN remained almost constant between 300¿ and 500¿C. Thermal curves for inorganic ions such as NH4+, NO3-, and SO42- were similar to those for CCN, whereas the thermal curve of sea salt particles and total carbon (organic plus soot) particles were different from CCN. The results suggested that inorganic materials such as ammonium sulfate and ammonium nitrate contributed greatly to the CCN in the urban atmosphere and ammonium sulfate contributed considerably to the CCN in the coastal atmospheres. Anthropogenic organic materials, which evaporated below 300¿C, contributed considerably to the CCN, whereas soot particles contributed hardly to the CCN.