The influence of CeO2 in the acceleration of NOx-assisted soot oxidation has been studied in flow-reactor equipment by comparing two catalyst configurations, namely: (1) Pt upstream of soot and (2) Pt upstream of CeO2-soot. The role of CeO2 has been elucidated by means of DRIFT spectroscopy coupled with mass spectrometry and TAP reactor experiments. It was found that CeO2 has the potential to accelerate the oxidation rate of soot due to its active oxygen storage. The formation of active oxygen is initiated by NO2 in the gas phase. A synergetic effect is observed as a result of surface nitrate decomposition, which results in gas phase NO2 and desorption of active oxygen. Stored oxygen is postulated to exist in the form of surface peroxide or super oxide. Active oxygen is likely to play a role on the acceleration of soot oxidation and to contribute more than desorbed NO2 or NO2 from surface nitrate decomposition. (C) 2004 Elsevier B.V. All rights reserved.