The light-off behavior for CO oxidation and NO reduction under stoichiometric CO + NO + O2 gas mixtures of a series of palladium catalysts supported on ceria–zirconia, ceria–zirconia/alumina and alumina has been examined by means of catalytic activity tests, and by DRIFTS, XANES and EPR spectroscopies. The results show the promoting effect of the Zr–Ce mixed oxide on both reactions. The extent of promotion depends on the characteristics of the promoter entities present for each catalyst and on the nature of the reaction. As observed in the absence of NO, CO oxidation was mainly enhanced by contact between Pd and 3D aggregated promoter entities, although an inhibiting effect of NO on CO oxidation, attributed to passivation of the interface sites due to formation of oxidized states of Pd, was observed at low temperatures. In contrast, the higher activity for NO reduction over catalysts supported on ceria–zirconia/alumina suggests that most active sites for that reaction involve Pd interacting with dispersed promoter entities.