The hydrogen permeation through surface modified Pd and Pd70Ag30 membranes has been studied at temperatures between 100 and 350 degreesC. Silver has been evaporated on Pd and Pd70Ag30 foils with a thickness of 25 mum in order to study the role of the surface composition in comparison with the membrane bulk composition. The Pd70Ag30-based membranes display the largest permeation rates at temperatures below 200 degreesC, while Pd membranes with 20 Angstrom silver evaporated on the upstream side show the largest permeation rates above 200 degreesC. There are, consequently, different rate limiting processes above and below 200 degreesC: at temperatures below 200 degreesC, the bulk diffusion through the membrane is rate limiting, while at temperatures above 200 degreesC, the influence of the surface composition starts to become significant. It has further been concluded that a sharp silver concentration gradient from the surface to the bulk is important for the hydrogen permeation rate at temperatures above 200 degreesC. Adding oxygen to the hydrogen supply will almost totally inhibit the hydrogen permeation rate when a pure Pd membrane surface is facing the upstream side, while for silver-containing surfaces the presence of oxygen has almost no effect. On a clean Pd surface, oxygen effectively consumes adsorbed hydrogen in a water forming reaction. With Ag on the surface, no water formation is detected. Co-supplied CO inhibits the permeation of hydrogen in a similar manner on all studied membrane surfaces, independent of surface silver content.