The efficiencies of hydrogen peroxide and oxygen as oxidants for the destruction of 3-chlorobiphenyl were compared under supercritical water conditions in the range of 473-723 K at a fixed density of 0.36 g/ml using a batch-reactor system. The effects of temperature, and oxidant concentration on the destruction of PCBs were also studied. The conversion of hydrogen peroxide was significantly higher than that of oxygen. The highest conversion was over 99.999%, and then the concentration of 3-chlorobiphenyl (3-PCB) in the collected effluent is only 0.3 ppb, while the conversion using oxygen was only 14.3% under the same conditions. Furthermore, in a flow-reactor system in addition to the batch-reactor system, supercritical water oxidation (SCWO) of polychlorinated biphenyls (PCBs) such as 3-PCB and Kanechlor KC-300 (KC-300) using hydrogen peroxide was carried out at 30 MPa and 673 K in the range of residence times from 10.7 to 101.7 s. In all runs, over 99% of 3-chlorobiphenyl was found to be decomposed perfectly by more than stoichiometric demand of hydrogen peroxide. The effect of residence times and the concentration of hydrogen peroxide have further been investigated.