Abstract: Assuming knowledge of the number and species of gases present within an enclosed environment such as the atmosphere of a containment vessel of a nuclear plant, an apparatus and process for receiving a gas sample from said atmosphere and measuring the percentages of the various known number and species of gases present is disclosed. Assuming N number of gas species present, N-1 discrete chambers are utilized. Each chamber confines the sample gas, includes a discrete permeation cell having an internal volume isolated from the rest of the chamber by a permeable membrane, and has a pressure sensor for measuring the pressure within the permeation cell resulting from gas permeating through the permeable membrane. Each chamber is isothermal and maintained at its own discrete temperature. Sample gas is introduced into each chamber and allowed to come to equilibrium with the temperature of the chamber. When thermal equilibrium is attained, the permeation cell in each chamber is evacuated by a vacuum pump.