Pyrene was used as a typical four-ring polycyclic aromatic hydrocarbon (PAH), to investigate the mechanisms and incineration behaviour of large organic molecules in a batch supercritical water oxidation reactor using hydrogen peroxide as oxidant. The distribution of carbon as gaseous species and organic species in relation to the temperature and pressure. and reaction time was monitored. The results showed that at 200 degrees C, pyrene was only slightly decomposed but as the temperature increased to 250 degrees C and then to 280 degrees C, carbonisation and thermal cracking became prevalent leading to char formation and decomposition of pyrene to phenanthrene. and later naphthalene. Rapid dissolution and oxidation of the char and organic species started occur-ring from 300 degrees C. Increasing reaction time resulted in increased formation of carbon dioxide and carbon monoxide. Initially high product formation of phenanthrene at short reaction times was followed by high decomposition of the organic products in solution as the reaction conditions became progressively more severe. Oxygenated organic species such as aldehydes, ketones, phenols, xanthone, and benzoic acid were identified as the temperature and reaction times were increased between 300 and 380 degrees C. From the analytical results obtained, carbon mass balances were calculated for each experiment. A proposed mechanism for the observed oxidative decomposition of pyrene is also reported. (c) 2005 Elsevier Ltd. All rights reserved.