The catalytic behaviours of CeO2 and a series of La3+-doped CeO2 catalysts (La3+ loading between 5 and 50 wt%) have been studied for soot oxidation by O-2. XRD and Raman spectroscopy characterisation indicated that solid Solutions are formed in the studied Ce/La ratio, in which La3+ cations replace Ce4+ cations in the CeO2 lattice. Thermogravimetric analysis showed that La3+ significantly improves CeO2 catalytic activity for soot oxidation with O-2. The best catalytic activity was found with 5 wt% La3+-doped CeO2 catalyst (CeO2-5La), in both loose and tight contact conditions. This improvement seems to be related to the increase in BET surface area and the change in the catalyst redox properties of CeO2 brought about by doping with La3+. La3+ decreases the onset temperature of Ce4+ to Ce3+ reduction by H-2 from 580 degreesC (CeO2) to 325 degreesC (CeO2)-5La) and increases the amount of Ce4+ that can be reduced by H-2 (maximum amount for CeO2-5La catalyst). An advanced TAP reactor is used for the first time to study catalysed soot oxidation with labelled oxygen. In the absence of catalyst, oxidation starts above 500degreesC, and mainly labelled oxidation species (CO and CO2)) were found. In the presence of catalyst, it is shown that the gas-phase labelled oxygen replaces nonlabelled lattice oxygen, creating the highly active nonlabelled oxygen. This highly active nonlabelled oxygen reacts with soot, giving CO and CO2. The creation Of Such active oxygen species starts front 400 degreesC and thereby decreases the soot oxidation temperature. CeO2-5La produces more such active species, for example, leading to 98% oxygen conversion at 400 degreesC compared with 37% over CeO2 alone under identical circumstances. (C) 2004 Elsevier Inc. All rights reserved.