AbstractThe photodecomposition of pendimethalin (N‐(l‐ethylpropyl)‐2,6‐dinitro‐3,4‐xylidine, l) in solution, as a thin film, and on soil surfaces under ultraviolet light and sunlight was investigated. Irradiation of pendimethalin in methanol yielded, in addition to the minor dealkylated product, the major products 2‐amino‐6‐nitro‐N‐(l‐ethylpropyl)‐3,4‐xylidine and 2‐nitroso‐6‐nitro‐3,4‐xylidine. Pendimethalin was found to degrade rapidly through reductive cyclisation of a nitro group and an adjacent N‐ethylpropyl group to give a cyclised benzimidazole product. The photodecomposition of pendimethalin involves oxidative dealkylation, nitro reduction and cyclisation.