Chemical stability for hydrogen sulfide of a platinum composite membrane. consisting of a platinum layer supported on a porous alumina tube by a CVD technique, was evaluated in comparison with a palladium composite membrane. These composite membranes gave high fluxes comparable to that of a reported palladium composite membrane prepared by an electroless-plating technique, as well as high ideal permselectivity for hydrogen over nitrogen, typically 240 for palladium and 210 for platinum at 773 K. When these composite membranes were in contact with gas stream including hydrogen sulfide, their hydrogen permeability declined rapidly. Many cracks were formed on the surface metallic layer of the palladium composite membrane, so that other gases besides hydrogen permeated mainly through the cracks formed. On the other hand, cracks were hardly formed for the platinum composite membrane. It was reported that the lattice constant of palladium was expanded from 0.39 to 0.65 nm by sulfidation of the metallic layer, but that of platinum was slightly changed from 0.39 to 0.35 nm. The difference in the expansion of lattice constant may affect structural change and rupture phenomena of these composite membranes. After the sulfurized platinum composite membrane was treated with pure oxygen flow, the hydrogen permeability was recovered up to 50% of that of the fresh membrane.