Gas composition and its evolution are studied in a volcanogenic basalt section of the northwestern Siberian Platform of Permian-Triassic age more than 2 km thick. It is shown that its major components are water and CO2, making up 80-95 % of the total mass of gases. Evolution of qualitative and quantitative composition of fluids is recorded in time as they were erupted in the range from Upper Paleozoic to Lower Mesozoic. Amount of the gas phase gradually decreases with water and CO2 being generally decreased but role of CO2 increased relative to water. The content of other fluid components is small and evolves insignificantly. General supply of H2O and CO2 gases to the surface is quite considerable and should be taken into account in paleoecological estimates.
