A method has been developed that can photoreduce polychlorinated biphenyl (PCB) to biphenyl with great speed and efficiency as well as at relatively low cost. This process uses visible light, generated by ordinary incandescent light bulbs, which is absorbed by a common dye sensitizer. The dye molecules, when excited by the absorption of light, can promote a chemical reaction between polychlorinated biphenyls and a hydrocarbon gas such as propane. In this chemical reaction, hydrogen is abstracted from the hydrocarbon gas molecule and is substituted for chlorine on the PCB molecule in a stepwise fashion, which ultimately yields the major reaction product biphenyl. This reaction occurs in a polar aprotic solvent at room temperature and is accelerated by the presence of an alkali metal hydroxide. The final residence of the chlorine appears to be a salt which precipitates from the reaction mixture. This procedure could be applied to the treatment of PCB contaminated transformer oils, soils, and landfill leachates. / M.S.