UV light irradiation of I-chloro-, 2-chloro-, 1,2-dichloro-, 2,3-dichloro-, and 2,7-dichlorodibenzo-p-dioxin in methanol leads to regioselective homolysis of carbon-oxygen bond from the singlet excited state to undergo rearrangement into chlorinated 2,2-biphenols. On the contrary, reaction from the triplet excited state of chlorinated dioxins results in selective formation of dechlorinated congeners without altering the dioxin skeleton.