The oxidative decomposition patterns of some aromatic and aliphatic hydrocarbons, which are components of crude and refined oil, have been studied in sub-critical and supercritical water conditions. The compounds include biphenyl, fluorene, hexadecane and eicosane. These compounds were oxidized in deionized water with hydrogen peroxide at temperatures of 300, 350, 370 and 380 degrees C, and at corresponding pressures of 10.5, 17.0, 2 1.0 and 22.5 MPa, respectively, using a batch stainless steel reactor. In one set of experiments, the reactions were stopped and quenched once the specified temperatures were reached. In another set of experiments, the reactions were held for a period of I h each at the designated temperatures. Results showed that the aromatic hydrocarbons were much more resistant to hydrothermal oxidation that the aliphatic ones. Fluorene was however more reactive than biphenyl as it was easily oxidized to 9-Fluorenone, which was the major intermediate Compound found. The fluorenone produced xanthone via epoxidation, while loss of CO gave biphenyl. Further oxidation produced compounds similar to those found during the oxidation of biphenyl itself. These compounds were mainly phenol, benzofuran, dibenzofuran, hydroxybiphenyls, acetophenone, benzaldehyde and benzoic acid. More complex decomposition patterns were observed for hexadecane and eicosane, involving both gas-phase and liquid-phase mechanisms. The major intermediate compounds were ketones and carboxylic acids, with small amounts of aldehydes. All the ketones bore their functional groups on carbon no. 2, indicating that they were formed by the oxidation of their corresponding 1-alkenes. Finally, the configuration of the reaction vessel was found to have affected the decomposition of these hydrocarbons especially the aliphatics. (C) 2007 Elsevier B.V. All rights reserved.