CeO2 catalysts have been prepared with conventional (Ref) and three dimensionally order macroporous (3DOM) structures, and the effect of the structure on the soot combustion mechanism has been studied in detail. Isotopic exchange experiments showed that the CeO2-3DOM catalyst produces more active oxygen upon O-2 chemisorption than the counterpart CeO2-Ref catalyst, and this active oxygen is transferred more efficiently to soot due to the macroporous structure. CeO2 -3DOM and CeO2-Ref also accelerate the oxidation of NO to NO2, and their activity is equal. However, CeO2-3DOM utilizes NO2 more efficiently than CeO2-Ref for soot combustion. NO2 has two roles in the soot combustion mechanism: i) reacts with soot and ii) is chemisorbed on ceria and produces active oxygen, which is more oxidizing than NO2. In ceria catalysts with a conventional structure, the main role of NO2 is the direct oxidation of soot, because active oxygen has restrictions to be transferred from catalyst to soot due to the poor soot-catalyst solid-solid contact. However, the 3DOM structure improves the transfer of active oxygen, and therefore, an additional benefit is obtained from NO2, that is, NO2 contributes to active oxygen production and the 3DOM structure allows its efficient transference to soot.