AbstractChlorothalonil (2,4,5,6‐tetrachloroisophthalonitrile) in benzene was photodecomposed by sunlight or low energy ultraviolet irradiation to a monophenyl adduct identified as 3,5,6‐trichlorobiphenyl‐2,4‐dicarbonitrile. However, irradiation with a 100‐W high‐energy ultraviolet lamp yielded an additional compound identified as 2′, 5′ ‐dichloro‐m‐terphenyl‐4′, 6′‐dicarbonitrile. Photolysis was followed by a steady decrease in the chlorothalonil and monophenyl adduct concentrations, with a concomitant increase in the diphenyl adduct.