The ceria-catalyzed soot oxidation mechanism has been studied by a pulse technique with labeled O-2 in the absence and presence of NO, using ceria-soot mixtures prepared in the loose contact mode. In the absence of soot, the ceria-catalyzed oxidation of NO to NO2 takes place with ceria oxygen and not with gas-phase O-2. However, the oxygen exchange process between gas-phase O-2 and ceria oxygen (to yield back O-2, but with oxygen atoms coming from ceria) prevailed with regard to the ceria-catalyzed oxidation of NO to NO2. Gas-phase O-2 did not react directly with soot when pulsed to a soot-ceria loose contact mixture. Instead, ceria oxygen is transferred to soot (this step does not require gas-phase molecular oxygen to be present), and gas-phase O-2 fills up the vacancies created on the oxide in a further step. The transfer of oxygen between ceria and soot occurred directly in the absence of NO. However, in the presence of NO, NO2 is expected to be additionally generated by ceria oxygen oxidation, which also reacts with soot. The main reaction products of the ceria-catalyzed soot oxidation reaction with NO/O-2 were CO2 and NO. Additionally, evidence of the reduction of NOx to N-2 was found. (C) 2012 Elsevier Inc. All rights reserved.