Hydrogen permeance decrease, owing to the covalent interaction of carbon monoxide with the Pd-Ag membrane surface, represents a considerable drawback, since this decreases the efficiency of the alloy membranes. This work proposes a novel Sieverts-Langmuir's model taking into account the mentioned adsorption. The proposed model equation, in order to take into account the fraction of the membrane surface not active for hydrogen permeation, introduces Langmuir's isotherm, which is the surface loading in the classical Sieverts' permeation equation. The evaluation of the alpha parameter and the Langmuir affinity constant involved in the proposed model was carried out using experimental data measured using a 60 mu m thick Pd-Ag commercial membrane at 647 K (374 degrees C), up to a total pressure of 700 kPa. The presented model yields, in a parametric form, a quantitative assessment of hydrogen permeance decrease caused by the adsorption of gases on Pd-Ag membrane surfaces. Therefore, this novel Sieverts-Langmuir's equation models hydrogen permeation through Pd-Ag-based membranes in the presence of inhibitor gases such as CO. In addition, by ab initio evaluation of the alpha and the Langmuir affinity constant, the proposed Sieverts-Langmuir's equation can identify a new membrane with better efficiency. (c) 2007 Elsevier B.V. All rights reserved.