The effect of CO and CO(2) on the performance and stability of Pd-Ag thin film membranes prepared by electroless plating deposition (EPD) was investigated, observing the presence of dissociation to carbon and oxygen which slowly diffuse in the membrane influencing also H(2) permeability. The effect of the two carbon oxides was investigated both separately and combined in the 400-450 degrees C temperature range over long-term cumulative experiments (up to over 350 h) on a membrane that already worked for over 350 h in H(2) Or H(2)-N(2) mixtures. An increase of the H(2) permeation flux was observed feeding only CO(2) in the range 10-20%. This effect was interpreted as deriving from the facilitated H(2) flux caused from oxygen diffusion (deriving from CO(2) dissociation) in the membrane. CO induces instead a partial inhibition on the H(2) flux deriving from the negative effect of CO competitive chemisorption as well as C diffusion in the membrane, which overcome the positive effect associated to oxygen diffusion in the membrane. Carbon and oxygen diffuse through the membrane with a rate two order of magnitude lower than hydrogen, and recombinate at the permeate side forming CO, CO(2) and CH(4) which amount increases with time-on-stream. The effect is reversible and not associated with the creation of cracks or defects in the membrane, as supported by leak tests. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.