The behavior of Pd-based systems supported on alumina and ceria-zirconia and the complex ceria-zirconia on alumina was investigated during light-off tests using a stoichiometric C3H6 + CO + NO + O-2 mixture. In situ XANES/DRIFTS studies of the Pd chemical state at the surface and bulk of the materials were conducted to determine the noble metal response to the gas atmosphere during light off. The initial, oxidized PdO-1ike phase was converted to Pd(0) in all cases except for the ceria-zirconia-supported catalyst. Contact between Pd and bulk ceria-zirconia particles appeared to stabilize Pd clusters in the 373-673 K region with an average Pd(I)-like oxidation state. The presence of hydrocarbon in the gas phase was essential in order to stabilize these Pd(l)-like species. The role of such a phase in providing a source of active oxygen for hydrocarbon oxidation is shown here to strongly decrease light off temperature and this, together with its stability over a wide range of temperatures, may offer a contribution to reducing hydrocarbon emissions during cold start. (C) 2003 Elsevier Inc. All rights reserved.