The use of co-deposition instead of sequential deposition during the preparation of Pd-Ag thin films by electroless plating deposition leads to two different nanostructures, e.g. a dendritic nanostructure or a more compact and dense film, allowing to analyze the role of this parameter, at equal membrane composition, on the performances. In pure H-2 the permeability to hydrogen of the second type of thin films is 3-4 times higher, but the presence of CO2 in the feed changes considerably the performances. The results are tentatively interpreted on the basis of a non-permanent in situ modification of the characteristics of the Pd-Ag thin films, with creation of strains and microholes particularly enhanced for the nanostructure present in the sample prepared by co-deposition. These strains and microholes are suggested to derive from the combined effect of CO2 (with creation of subsurface O and/or C) and of hydrogen diffusion through the thin film, which induces lattice expansion and stress on the nanograins. When the flux of H-2 stops, there is a relatively rapid restoring of the initial situation. Scanning electron microscopy (SEM) characterization after the tests in the presence of CO2 indicates the presence of desintering consistently with above indications and the creation of crack like voids. (C) 2010 Elsevier B.V. All rights reserved.