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This is the first report of the dissolved gas composition of a deep-sea venting hydrothermal system originating from arc volcanism; the origin of dissolved gases in hydrothermal fluids from the Suiyo seamount in the southern Izu-Bonin (Ogasawara) Arc is discussed on the basis of chemical and isotopic compositions. Twelve high-temperature (up to 310°C) fluid samples were collected from three vents of the hydrothermal system using the submersible Shinkai 2000. The average 3 He 4 He ratio of the dissolved He is 8.1 ( R R atm ), which is within the range of values reported for N-MORB. The CO 2 3 He ratio is 12 × 109, which is within the range of arc volcanic gas values. The δ13C values of CO2 and CH4 are −1 and −8.5‰ respectively, which is quite high compared with other subaerial arc volcanic gases and those from mid-ocean ridges. These results, which do not accord with those from subaerial volcanism, are unlikely to result from fractionation or addition of carbon sources during the fluid circulation, but are regarded as characteristic of the magma source of Suiyo Seamount. A simple mixing of 97% MORB mantle with 3% subducting slab explains well the composition of the helium and carbon in the parent magma.