Abstract: The invention concerns a method and a device for purifying a gas containing hydrogen isotopes, and for recovering the different isotopes, in particular for treating a gas coming from a thermonuclear fusion reactor. According to the invention, the gas is firstly dried in a water and CO.sub.2 adsorption stage (2), the dried gas then passes through a stage (3) in which a selective adsorption of sulphur compounds takes place and finally, the gas leaving this stage, which still contains at least some of the compounds (H,D,T).sub.2, N.sub.2, O.sub.2, CO, C(H,D,T).sub.4, is processed in a hydrogen adsorption/separation stage (4) of the zeolite type.