Pd-Ni catalysts supported on Al2O3 and (Ce,Zr)O-x/Al2O3, both in fresh and thermally aged states, were examined with the main aim of determining the effects of Ni and Ce-Zr mixed oxide on the catalytic activity for CO and C3H6 oxidation, and NO reduction reactions under stoichiometric conditions. For this purpose, catalytic activity results were analysed in conjunction with DRIFIFS and XANES spectra recorded under reaction conditions to obtain information on the processes occurring in the catalysts during the course of the reactions. While a nickel-induced promotion of the formation of contacts between palladium and the Ce-Zr mixed oxides generally enhances the CO oxidation performance of the catalyst, it results detrimental to the NO reduction activity. This has been related to the higher difficulty in achieving the most active metallic palladium state for comparatively similar particle sizes in the presence of such contacts. Comparison of DRIFTS spectra under various reactant mixtures and between fresh and aged systems was shown to explain in a unique way the detrimental catalytic effects of hydrocarbon self-poisoning and sintering of the active components in the catalysts. (c) 2006 Elsevier B.V. All rights reserved.