Pd-Cu composite membranes were made by successive electroless deposition of Pd and then Cu onto various tubular porous ceramic supports. Ceramic filters used as supports included symmetric alpha-alumina (nominal 200 nm in pore size), asymmetric zirconia on alpha-alumina (nominal 50 nm pore size), and asymmetric gamma-alumina on alpha-alumina (nominal 5 nm pore size). The resulting metal/ceramic composite membranes were heat-treated between 350 and 700degreesC for times ranging from 6 to 25 days to induce intermetallic diffusion and obtain homogeneous metal films. Pure gas permeability tests were conducted using hydrogen and nitrogen. For an 11 mum thick, 10 wt.% Cu film on a nominal 50 nm pore size asymmetric ultrafilter with zirconia top layer, the flux at 450 degreesC and 345 kPa H-2 feed pressure was 0.8 mol/m(2) s. The ideal hydrogen/nitrogen separation factor was 1150 at the same conditions. The thickness of the metallic film was progressively decreased from 28 mum down to 1-2 mum and the alloy concentration was increased to 30 wt.% Cu. Structural factors related to the ceramic support and the metallic film chemical composition are shown to be responsible for the differences in membrane performance. Among the former are the support pore size, which controls the required metal film thickness to insure a leak-free membrane and the internal structure of the support (symmetric or asymmetric) which changes the mass transfer resistance. The support with the 200 nm pores required more Pd to plug the pores than the asymmetric membranes with smaller pore sizes, as was expected. However, leak-free films could not be deposited on the support with the smallest pore size (5 nm gamma-alumina), presumably due to surface defects and/or a lack of adhesion between the metal film and the membrane surface.