Thin (less than 2 mum thickness) and pinhole-free palladium-copper (Pd-Cu) alloy composite membranes with a diffusion barrier have been fabricated on mesoporous stainless steel supports by vacuum electrodeposition. The deposition film was fabricated by multilayer coating and diffusion treatment and the formation of Pd-Cu alloys was achieved by annealing the as-deposited membranes at 723 K in nitrogen atmosphere. To improve the structural stability of Pd alloy/Ni-SUS composite membranes, a thin intermediate layer of silica by sol-gel method was introduced as a diffusion barrier between Pd-Cu active layer and a modified SUS substrate. The composition and phase structures of the alloy film were studied by energy dispersive electronic analysis (EDS) and X-ray diffractometry (XRD); the typical Pd-Cu plating had composition of 63% Pd and 37% Cu and the atomic inter-diffusion of Pd and Cu resulted in Pd-Cu alloys in a fcc structure. The electron probe microanalyzer (EPMA) profiling analysis indicated that the improved membranes were structurally stable. The Pd-Cu alloy composite membrane obtained in this study yielded excellent separation performance for hydrogen: hydrogen permeance of 2.5 x 10(-2) cm(3)/cm(2) cmHg s and hydrogen/nitrogen (H-2/N-2) selectivity was above 70 000 at 723 K.