A novel hybrid process of hydrothermal or supercritical water oxidation and TiO2 photocatalysis was developed to examine the degradation of chlorobenzene as a model of the oxidative decomposition of organic pollutants. Aqueous solutions of chlorobenzene containing H2O2 as the oxidizing agent and/or colloidal TiO2 nano-particles as catalyst, were fed into the reactor with the temperature and the pressure controlled to be T = 25-400 degrees C and P = 30 MPa, respectively. Chlorobenzene was considerably decomposed in the presence of H2O2 under hydrothermal conditions for T >= 300 degrees C. It appeared that photocatalytic decomposition of chlorobenzene takes place at all temperatures by colloidal TiO2 nanoparticles under irradiation with near-UV light. We have realized the synergic decomposition of chlorobenzene by the coexistence of H2O2 and TiO2 in which maximum conversion is more than 80% under irradiation at T - 200 degrees C.