Hydrogen permeation measurements were carried out to investigate how a co-existing gas with hydrogen affects the hydrogen permeability of a silver 23 wt%-palladium alloy of 20 mu m in thickness. The hydrogen flux was reduced by the co-introduction of water vapor, carbon dioxide, carbon monoxide or methane. The lower the temperatures at which the measurements were carried out, the more obvious were the reductions of the hydrogen fluxes. An empirical rate equation of the surface reaction was obtained by a precise analysis of the results of hydrogen permeation measurements. The rate constant was found to depend on temperature, partial pressure and the components of the co-existing gas. Performance simulation was carried out utilizing the obtained rate equation of the surface reaction. It suggests that an interference effect of the co-existing gas is negligible at temperatures above 873 K for the membranes thicker than 10 mu m at 873 K. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.