Separation of protium and deuterium using a palladium membrane was investigated under temperature and pressure conditions involving a phase transformation of the metal. The H/D flux ratio for single-component systems showed a maximum of 7-8, when the membrane surfaces on the supply and permeate sides were in the beta- and alpha-phases, respectively, for protium, and when both surfaces were in the alpha-phase for deuterium. The large difference in solubility of protium and deuterium in palladium was responsible for the high selectivity. Fora mixture of H-2 and D-2, the H/D ratio in palladium was expressed by an equation similar to Raoult＇s law. The H and D fluxes were lower than those for single component systems, and the H/D separation coefficient was 1.2-1.3.