Three PdO/gamma-Al2O3 catalysts differing in Pd loading (between 0.05 and 1 Pd wt%) have been examined with regard to their light-off catalytic activity for CO oxidation and NO reduction reactions under stoichiometric conditions. Catalytic activity results are explained on the basis of DRIFTS analysis of the adsorbed species present under reaction conditions. It is shown that differences between the catalysts (apart from the expected increasing activity with the Pd loading for both reactions) are considerably greater for NO reduction than for CO oxidation reactions. This is explained by structural differences between the active metallic Pd particles formed during the particle nucleation/growth process that takes place during the course of the light-off run upon interaction with the reactant mixture. On the basis of differences in the natures and relative intensities of adsorbed CO and NO species present during competition for atop and bridging sites over the Pd particles, the correlation (within the Pd loading studied) between NO reduction capability and Pd loading is attributed to the increasing NO dissociation efficiency as the relative size of the particles formed during the course of the reaction is increased. (C) 2003 Elsevier Inc. All rights reserved.