In order to investigate the oxidative reaction pathways of dicarboxylic acids under hydrothermal condition, sebacic and azelaic acids of high molecular weight dicarboxylic acids were oxidized at 400 degrees C, and intermediate products were identified in detail by GUMS, HPLC, and H-1 NMR analyses. The oxidation of high molecular weight dicarboxylic acids proceeded with the consecutive oxidation of higher to lower molecular weight dicarboxylic acids through alpha-, beta-, and gamma-oxidations, as do monocarboxylic acids. The oxidation of dicarboxylic acids was also accompanied by oxidative decarboxylation, which led to the formation of monocarboxylic acids. (C) 2006 Elsevier B.V. All rights reserved.