The hydrogen permeation mechanism through a supported platinum membrane prepared by chemical vapor deposition (CVD) was investigated and compared with an electroless-plating platinum membrane. A CVD membrane while maintaining an excellent hydrogen permselectivity provided a 10(3)-10(4) times higher hydrogen permeation coefficient than an electroless-plating platinum membrane. This large difference in the hydrogen permeation coefficient resulted from the hydrogen transport phenomena, namely the surface diffusion mechanism for the CVD membrane and solution-diffusion mechanism for an electroless-plating membrane. Furthermore, the relationship between the structure of the metal layer and hydrogen permeance was also discussed for the CVD membrane. The membranes with smaller platinum grains gave higher hydrogen fluxes because the effective platinum area for the hydrogen surface diffusion increased with a decrease in grain size. (C) 2000 Elsevier Science B.V. All rights reserved.