The influence of H2S-to-H-2 partial pressure ratio on the sulfidization of Pd and 70 mol% Pd-Cu membrane alloys was studied using various H2S-containing gas mixtures. The Pd membranes exposed to various H2S mixtures were in very good agreement with the thermodynamic calculations used in this study, resisting sulfidization when exposed to H2S-to-H2 ratios below the equilibrium value predicted for Pd4S formation, and experiencing sulfidization when exposed to ratios above the equilibrium values. The 70 mol% Pd-Cu membranes, however, exhibited deviations from the predicted values, resisting sulfidization at some conditions close to the equilibrium values at which sulfidization was expected, and experiencing sulfidization at some conditions at which resistance was expected. This phenomenon was attributed to deviations of the Pd-Cu alloy from ideality, probably due to Cu segregation at the membrane surface. (c) 2007 Elsevier B.V. All rights reserved.