Amongst the environmental chemical groups polycyclic aromatic hydrocarbons (PAH) comprise the largest group of carcinogens. Hydrothermal oxidation is an emerging technology which has attracted attention for the destruction of organic Compounds in environmental samples. The oxidation reactions occur in an aqueous fluid phase under conditions below and around the critical point of water (T-c = 374 degrees C, P-c = 22.1 MPa). In this paper polycyclic aromatic compounds have been oxidised in a hydrothermal oxidation batch reactor. The PAH investigated were naphthalene and phenanthrene. The influence of temperature, pressure, sub-critical and supercritical conditions and reaction time were investigated in relation to the mechanisms of destruction of the PAH. At lower temperatures the PAHs were thermally cracked but as the temperature increased, hydroxylation of the aromatic moiety occurred leading to a series of activities including ring-opening and rearrangement reactions. It was found that up to 99 wt.% destruction of the PAH occurred at supercritical conditions. (C) 2006 Elsevier B.V. All rights reserved.