The role of NO2 and O-2 in the combustion of soot was investigated by analysing the soot combustion characteristic and the DRIFT spectra of surface oxygen complexes (SOCs) as soot oxidation intermediates. It is found that in NO2/O-2-soot, like in the CRT system, the oxidation of soot is initiated by the NO2-soot reaction to create SOCs as intermediates. O-2 reacts with SOCs, eventually yielding CO and/or CO2. This mechanism results in a higher oxidation rate when NO2-soot reaction occurs in the presence of O-2. The potential of using Printex-U, a flame soot, in exploratory diesel soot combustion studies is discussed. It is concluded that in general this is the case except for the fact that real soot can contain adsorbed hydrocarbons that will increase the reactivity of the soot in the absence of NO2. (C) 2004 Elsevier B.V. All rights reserved.