Na/Al2O3 catalysts are tested in the soot oxidation reaction using O-2 and NO/O-2 as oxidising agents. The activity of the catalysts . varies with oxidising agent and the presence of surface SO42-.. NO(g) increases conversion of soot to COx-. Surface SO42- has no effect on activity for the C-(s) + O-2 reaction but decreases activity in the Carbon + NO/O-2 oxidation. This is interpreted in terms of SO42- blocking NO adsorption sites, thus poisoning the 4 NO + O-2 --> NO2 reaction, which is essential for the NOx promoted oxidation.