Effects of Ag concentration (0-45 at.%) on the hydrogen permeation of Pd-Ag coated V-15Ni composite membranes were investigated in the temperature range of 423-673 K, using gas permeation technique. Dense and uniform Pd-Ag alloy overlayer was obtained using a DC multi-target sputtering system. At temperatures higher than 473 K, the hydrogen permeation through the V-15Ni/Pd-Ag composite membranes was mainly bulk-diffusion limited and had little dependence on the overlayer composition. At 473 K and lower, the addition of Ag to the overlayer obviously increased the hydrogen permeability of the V-15Ni/Pd-Ag composite membranes. The hydrogen permeability of Pd-30Ag coated composite membrane was 2.53 x 10(-8) mol H-2 m(-1) s(-1) Pa-1/2 at 473 K, which was about 27% higher than that of Pd-coated composite membrane. The resistance of overlayer to hydrogen-induced cracking increased with the addition of Ag. Effects of the interfacial contaminants on the hydrogen permeation of the composite membrane were discussed. (C) 2003 Elsevier B.V. All rights reserved.