Thin and defect-free Pd-Cu composite membranes with high hydrogen permeances and selectivities were prepared by electroless plating of palladium and copper on porous alumina supports with pore sizes of 5 and 100 nm coated with intermediate layers. The intermediate layers on the 100 nm supports were prepared by the deposition of boehmite sols of different particle sizes, and provided a graded, uniform substrate for the formation of defect-free, ultra-thin palladium composite layers. The dependence of hydrogen flux on pressure difference was studied to understand the dominant mechanism of hydrogen transport through a Pd-Cu composite membrane plated on an alumina support with a pore size of 5 nm. The order in hydrogen pressure was 0.98, and indicated that bulk diffusion through the Pd-Cu layer was fast and the overall process was limited by external mass-transfer or a surface process. Scanning electron microscopy (SEM) images of the Pd-Cu composite membrane showed a uniform substrate created after depositing one intermediate layer on top of the alumina support and a dense Pd-Cu composite layer with no visible defects. Cross-sectional views of the membrane showed that the Pd-Cu composite layer had a top layer thickness of 160 nm (0.16 mu m), which is much thinner than previously reported. (C) 2011 Elsevier B.V. All rights reserved.