A series of monometallic Pd and bimetallic Pd-Cr catalysts supported on CeOx/Al2O3 (CA) and (Ce, Zr)O-x/Al2O3 (CZA) materials have been characterized using a combination of X-ray diffraction (XRD), electron microscopy, and Raman spectroscopy, and employing electron paramagnetic resonance and in situ diffuse reflectance Fourier transform infrared and X-ray near-edge structure spectroscopies to analyze the redox and chemical processes taking place under light-off conditions in a gaseous, stoichiometric mixture containing CO, NO, and O-2. The catalytic behavior of these mono and bimetallic systems was strongly affected by the nature of the cerium-containing promoter. The Pd-ceria interface appeared significantly more active than Pd-ceria/zirconia in both CO oxidation and NO reduction processes under stoichiometric conditions. This factor seems to dominate the differential behavior of Pd monometallic systems. In the case of Pd-Cr bimetallic systems, an additional strong influence of the base metal on the oxidation state of Pd appears of importance in explaining the catalytic behavior of the samples. For the CeOx/Al2O3 supported catalyst, the formation of Pd(0) at lower temperatures leads to an initial enhancement in activity, while formation of a Pd-Cr alloy above 473 K suppressed full NO conversion at high temperatures. These effects were absent in the (Ce, Zr)O-x/Al2O3 supported system. (C) 2003 Elsevier B.V. All rights reserved.