A novel hybrid process of hydrothermal or supercritical water oxidation and TiO 2 photocatalysis was developed to examine the degradation of chlorobenzene as a model of the oxidative decomposition of organic pollutants. Aqueous solutions of chlorobenzene containing H 2 O 2 as the oxidizing agent and/or colloidal TiO 2 nanoparticles as catalyst, were fed into the reactor with the temperature and the pressure controlled to be T = 25-400°C and P = 30 MPa, respectively. Chlorobenzene was considerably decomposed in the presence of H 2 O 2 under hydrothermal conditions for T ≧ 300°C. It appeared that photocatalytic decomposition of chlorobenzene takes place at all temperatures by colloidal TiO 2 nanoparticles under irradiation with near-UV light. We have realized the synergic decomposition of chlorobenzene by the coexistence of H 2 O 2 and TiO 2 in which maximum conversion is more than 80% under irradiation at T = 200°C.