Palladium acetate was sublimed at a reduced pressure at 400℃, carried into the macropores of the porous wall of an α-alumina support tube and was decomposed there. A thin palladium membrane which was thus formed showed a hydrogen permeance of 10^-6 mol m^-2 s^-1 Pa^-1 and a hydrogen/nitrogen permselectivity higher than 1000. The membrane was stable against hydrogen embrittlement even when the permeation temperature was varied between 100 and 300℃, and it was stable to sulfur or chlorine. To test the ability of this system for the separation of hydrogen and deuterium, a palladium disk was used instead of the prepared membrane since a definite membrane thickness was necessary for calculation. When H₂ and D₂ permeated through the membrane independently, the H/D permselectivity was appro- ximately 7 at 150-200℃ under a feed side pressure of 0.4 MPa and a permeate side pressure of 0.1 MPa. When a mixture of H₂ and D₂ was fed, the H/D permselectivity was reduced to 1.2-1.6.