For the environmental degradation of polychlorinated biphenyls (PCB's) decomposition by sunlight may become the major pathway of degradation owing to their chemical stabilities. It is therefore important to establish the extent to which photodegradation occurs and to identify the photodegradation products for environmental safety. Under ultraviolet irradiation at 254 nm, 300 nm, or 310 nm wavelengths, PCB's were significantly decomposed and the photodegradation products were mainly less chlorinated biphenyls, caused by dechlorination reaction, in cyclohexane, hexane, or isooctane solutions, and methoxybiphenyls, caused by replacement reaction, in methanol solution. In the present paper, the photodegradation of decachlorobiphenyl (DCB, I) under xenon lamp irradiation in the range of 199 nm to 358 nm wavelengths and its photodegradation products were studied.