The behavior of Pd three-way catalysts promoted with Ce-Zr mixed oxides and supported on alumina subjected to thermal degradation treatments has been examined during light-off tests under stoichiometric CO+NO+O-2. Attention was paid to the region of aging temperature 1273-1373 K, where phase segregation of the ceria-zirconia promoter occurs. Characterization of the samples was performed using X-ray diffraction and Raman spectroscopy. in situ XANES/DRIFTS studies of the Pd chemical state at the surface and in the bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light-off. Electron paramagnetic resonance was used to analyze the promoter surface and to study the redox response of the materials in contact with the reactant gases. Thermal degradation appears to sinter both the noble metal and promoter components of the catalyst but also stabilizes oxidized Pd entities in contact with the promoter prior to the occurrence of the Ce-Zr phase segregation reaction. Additionally, it enriches the promoter surface in the lanthanide cation. The influence of these physicochemical phenomena on the catalytic properties of the Pd-based TWC is discussed. (C) 2003 Elsevier Inc. All rights reserved.