The effect of external mass transfer, competitive adsorption and coking on hydrogen permeation through thin Pd/Ag membranes has been evaluated by means of comparisons between experimental and predicated data at temperatures of 548-723 K and a total pressure of 2 bar. Predicted data were obtained by solving a mathematical model taking into account the effect of external mass transfer on hydrogen permeation only and verified from permeation experiments with a mixture feed of H-2-N-2. It was found that steam and CO have significant inhibitive effects on hydrogen permeation through the membrane due to their competitive adsorption with hydrogen on the metallic surface at temperatures less than 623 K, whereas, CO2 displayed only slight inhibitive effect at these temperatures. The effect increased with the amount of steam, CO or CO2 in the mixed feed, and decreased with temperature. Only very slight effects from steam and CO were noted at temperatures higher than 673 K. Experiments with a mixed feed of hydrogen and methanol confirmed that methanol in the feed had a significant impact on hydrogen permeation as the membrane surface was contaminated and partly covered by a carbonaceous deposit produced from the decomposition of methanol even at a very low concentration of 1% methanol in the mixed feed. On the other hand, if steam was present in the mixed feed the membrane performance could be maintained by oxidation of the carbonaceous deposit into CO or CO2.