CO adsorption has been studied over Pd/Al2O3 and Pt/Al2O3 catalysts using IR spectroscopy in different conditions and after different pretreatments. The oxidation of CO by oxygen and water (water gas shift) has also been studied at 130-673K. The spectra show that the most active species as oxidant for CO are dispersed Ptn+ and Pdn+ cations and PdOx species mainly "decorating" the defects of alumina crystals where they modify completely the behaviour of surface OH's and Lewis acid sites. Some of these species oxidize CO to CO2 already below similar to 200 K. These highly oxidized species escape detection by IR during reaction at 500-673 K, when only reduced sites are apparent due to their very strong interaction with CO. The data suggest that redox mechanisms could occur for both WGS and CO oxidation occurring on highly dispersed cationic species non detectable under reaction conditions. (C) 2011 Elsevier B.V. All rights reserved.