AbstractDiafenthiuron, 1‐tert‐butyl‐3‐(2,6‐di‐isopropyl‐4‐phenoxyphenyl)thiourea, is an effective insecticide and acaricide. Sunlight degradation of diafenthiuron in various aqueous solutions and pure hexane yielded two major identified products: 1‐tert‐butyl‐3‐(2,6‐di‐isopropyl‐4‐phenoxyphenyl)carbodiimide (CGA‐140 408) and 1‐tert‐butyl‐3‐(2,6‐di‐isopropyl‐4‐phenoxy‐phenyl)urea (CGA‐177 960). CGA‐140 408 was further photo‐transformed into CGA‐177 960 by sunlight. Direct photolysis appeared to be a major photolysis pathway of diafenthiuron in the environment. Photodegradation of CGA‐140 408 and CGA‐177 960 was enhanced in humic acid water, paddy water and aqueous acetone solutions, and followed first‐order kinetics. Isopropanol (a radical quencher) and de‐aeration strongly inhibited the photolysis of these chemicals, which suggested oxygen radical‐mediated reactions.© 2002 Society of Chemical Industry