An advanced TAP reactor is used for the first time to study CeO2 catalysed soot oxidation using labelled oxygen. In the absence of catalyst oxidation takes place above 500degreesC and mainly labelled oxidation species (CO and CO2) were observed. In the presence of catalyst it is shown that the gas-phase labelled oxygen replaces non-labelled lattice oxygen of CeO2 creating highly active oxygen. This highly active non-labelled oxygen reacts with soot giving CO and CO2. The creation of such active oxygen species starts from 400degreesC and it will decrease the soot oxidation temperature. The rate of gas-phase oxygen exchange by the CeO2 lattice oxygen and the rate of this very active lattice oxygen with soot are much faster than the reaction rate of O-18(2) reacting with soot directly. Similarly the rate of active oxygen reaction with soot is much faster than the rate of its combination giving gas-phase O-2.