Abstract The hydrogen permeance of Pd 0.75 Ag 0.25 membranes was measured in the presence of pure H 2 or mixtures of H 2 with CO and H 2 O at temperatures in the range of 300–773 K and at atmospheric pressure under gas-driven permeation (GDP) and plasma-driven permeation (PDP). The Arrhenius plots of the permeability through these membranes versus the inverse temperature showed a small peak in the intermediate temperature range and different activation energies in the low and high temperature ranges. The experimental data also indicated a more pronounced effect of CO on the permeability than H 2 O, and a similar effect of GDP and PDP. The stronger inhibition by CO was due to the strong interaction between CO and Pd, and this effect was eliminated at temperatures higher than 623 K.