A theoretical model is proposed to describe hydrogen permeation in palladium and silver-palladium membranes in presence of a non-inert gas as CO, it is known indeed that hydrogen flux through palladium-based membranes drastically decreases when H(2) is fed in mixtures containing carbon monoxide due to the interaction of the latter gas with the membrane surface. To model this process, the adsorption step of the well-known approach suggested by Ward and Dao has been suitably modified, since it must be considered as a competitive adsorption of the different non-inert molecules on the metal interface. In particular, the competitive adsorption of CO and H(2) has been examined accounting for the spectrum of information available for CO adsorption on palladium, as well as for hydrogen in palladium and palladium-silver alloys. A validation of the model proposed has been performed through a comparison between several literature data and model calculations, over a rather broad range of operating conditions. A quire good agreement was obtained in the different cases, the model, thus, can be profitably used for predictive purposes (C) 2010 Elsevier B V. All rights reserved.