Hydrogen permeation measurements were carried out by using an Ag 23 wt%-Pd alloy membrane of 20 put thickness at temperatures from 773 to 873 K. Contrary to some reported results, hydrogen permeation flux was not affected by the co-existence of H2O up to 9.5% at 873 and 773 K under non-pressurized conditions. Further addition Of CO2 up to 27.9%, CH4 up to 6.8%, CO up to 4.6% did not reduce the permeation flux. Similarly, permeation flux was not affected in a pressurized system with H2O up to 63%, 5% of CO with 30% of H2O, 20% of CH4 with 15% of H2O, under 0.29 MPa pressurization at 873 K. From the hydrogen partial pressure dependence of the permeation flux, the rate determining step was concluded to be the bulk diffusion of H in the membrane under the abovementioned conditions. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.