We report first data on chemical composition of the gas emitted by the geothermal system of Sousaki, Greece. Gas manifestations display typical geothermal gas composition with CO2 as the main component and CH4 and H2S as minor species. Soil gas composition derives from the mixing of two end-members (atmospheric air and geothermal gas). Soil CO2 fluxes range from<2 to 33,400 g m-2 d-1. The estimated diffuse output of hydrothermal CO2, estimated for an area of 0.015 km2, is about 630 g s-1, while a tentative estimation of CH4 diffuse output gave a value of about 1.15 g s-1. Point sources accounted for lower flux values of ~26 g s-1 of CO2, ~0.1 g s-1 of CH4 and ~0.02 g s-1 of H2S.