The aim of this study was to investigate the oxidation of two different engine oils: a mineral and a synthetic engine oil. The mineral oil was a CEC3 reference oil often used in the CEC fuel tests. The synthetic oil was biologically degradable often used for fuel tests. The oils were analyzed by liquid-state C-13 NMR, H-1 NMR and cyclic fast neutron activation analysis (cFNAA) and were oxidized at about 550degreesC at atmospheric pressure in an oxidizer with constant airflow. The amount of oxygen in the condensed oil after oxidation and soot produced was measured using cFNAA. The soot produced by the mineral oil at 550degreesC was studied by solid-state C-13 NMR.The amount of oxygen in the mineral oil was estimated to 1.6 +/- 0.2 wt.% and in the synthetic oil it was 10.1 +/- 0.1 wt.%. After oxidation at 550degreesC, the synthetic oil produced almost 25% more soot than the mineral oil. The amount of oxygen in the soot produced by the mineral oil was 24.9 +/- 0.5 wt.% and in the soot derived by the synthetic oil it was 23.4 +/- 0.5 wt.%. Seemingly, the degradation of the mineral engine oil goes via the formation of oxidized aliphatics. Interestingly, the soot derived from the mineral oil at 550degreesC was similar to combustion chamber deposits (CCDs) derived from the same oil in a previous study in terms of oxygen content and chemical composition. (C) 2003 Elsevier B.V. All rights reserved.