Effects of diesel soot composition and accumulated vehicle mileage on soot oxidation characteristics were examined. Four soot samples were extracted from the crankcase oils of diesel engines that had accumulated different mileages. Carbon black was used as a comparative example. Soot structure was studied in situ using X-ray diffraction as it was oxidized to temperatures as high as 700 degrees C. The soot from older engines exhibited a higher increase in lattice spacing (d(002)) with an increase in temperature that resulted in soot samples being at lower temperatures, thereby reducing the oxidation resistance. Composition of the residues after oxidation was studied using X-ray diffraction, energy-dispersive spectroscopy, and X-ray absorption near edge structure. Oxidation residue of the soot samples is made up of decomposed lubricant additives compounds and debris from wear and tribofilms. XRD phase analysis showed that crystalline compounds in soot are CaSO4, CaZn2(PO4)(2), Ca-3(PO4)(2), Zn-3(PO4)(2), and ZnO. The turbostratic structure of all the soots irrespective of engine age is similar prior to oxidation; however, the embedded crystalline and amorphous species in the soot change with accumulated mileage. Surface area of the soot measured using BET was found to be inversely proportional to the weight of residue.