[en] Tritium retention in the vacuum vessel emerged as a potentially serious constraint in the operation of the International Thermonuclear Experimental Reactor (ITER) several years ago. Present-day outlook reveals that the main tritium amount will be retained in deposits in the divertor area and by repeatedly applied proper thermal treatment its amount would never exceed the permitted critical mass. During such heat treatments, the vacuum vessel will be heated, too, and it may represent a tritium sink, which is highly undesirable, as the kinetics of the tritium absorption rate is not known. The presented results on permeation through AISI 316 ITER grade stainless steel membranes in the pressure range between 50 and 1000 mbar at temperatures from 200 °C to 400 °C match well with reported values on AISI 316 steels. The accent is on the influence of the residual atmosphere on achieved permeability during heat treatments at moderate temperatures.