Photocatalytic degradation of polychlorinated biphenyls (PCBs) in a plate-type batch recycle photoreactor using a TiO2 thin film is experimentally examined in an acetonitrile/H2O system. The concentrations of higher chlorinated biphenyls, hexa-, hepta- and octa-CBs, degrease regularly with irradiation time; those of lower chlorinated biphenyls, tetra- and penta-CBs, increase in the initial period, and then decrease slowly. It is suggested that some series reactions occur from higher chlorinated biphenyls to lower ones. Based on the analysis of mass fragmentograms, various PCBs decomposition/production reactions are found to occur. The extend of dechlorination rate of total PCBs from tetra- to octa-CBs is approximately 35 % after 12 hours of irradiation through providing oxygen gas.