In this work, an attempt was made to identify and decouple several effects contributing to the complexity of kinetics of diesel particulate matter oxidation by oxygen. Adsorbed hydrocarbons provided limited contribution to the overall reactivity of the un-pretreated particulate matter. Even upon their thermal desorption, the first few percents of carbon in particulate matter were oxidized at an anomalously high rate. Further oxidation occurred at a lower rate with rather uniform kinetic characteristics. The high initial reactivity can be restored repeatedly by exposure of particulate matter to the ambient air for the prolonged periods of time (weeks) at room temperature. The experimental results suggest that these changes in reactivity of carbon are chemical, not morphological, in nature. The results are consistent with the hypothesis that prolonged ambient aging leads to formation of some highly reactive groups on the carbon surface, responsible for the subsequent relatively facile initial oxidation. A high density of these surface groups presumably cannot be obtained during the more vigorous oxidation process at higher temperatures. (C) 2003 Elsevier B.V. All rights reserved.