Oxidative activities of ten different catalysts to particulate matter (PM) emitted from it diesel engine have been compared by thermogravimetric analysis under an atmosphere containing O-3 and NO2. MnO2 has been found to exceed noble metal catalysts in the PM oxidation efficiency even at 300 degrees C. The PM oxidative activities of transition-metal oxides possessing high formation enthalpies are enhanced by the active oxygen species owing to acceleration of redox catalytic cycles.