The catalytic and photocatalytic hydrothermal oxidation of chlorobenzene has been examined at reaction temperatures ranging from 25 to 400 degrees C and at a fixed pressure of 30 MPa, using Cu(II)-grafted tungsten trioxide (Cu/WO3) as the catalyst and hydrogen peroxide as the oxidizing reagent. The decomposition efficiency of chlorobenzene is dramatically promoted with the aid of Cu/WO3, and approximately 100% decomposition efficiency has been achieved in the dark at a reaction temperature of 200 degrees C. It is proposed that the formation of hydroxyl radicals from hydrogen peroxide via the Fenton-type reaction catalyzed by Cu ions is subjected to substantial acceleration under subcritical conditions of water at around 200-300 degrees C, resulting in enhancement of the decomposition efficiency. Irradiation of catalysts with near-UV visible light also facilitates the decomposition of chlorobenzene because of the photocatalytic action.