AbstractAcifluorfen was irradiated in pure water at various excitation wavelengths and pH values. Numerous photoproducts were obtained which were identified by [1H]NMR and /or HPLC–MS/MS. The main reaction pathways were photo‐decarboxylation, photo‐cleavage of the ether bonding with formation of phenolic compounds, photo‐dechlorination and photo‐Claisen type rearrangement. Decarboxylation was observed in acidic and neutral media whereas cleavage of the ether bonding dominated in basic media. The photo‐Claisen type rearrangement only occurred on excitation at short wavelengths. The quantum yield of photolysis was significantly lower at 313 nm (6.1 × 10−5) than at 254 nm (2.0 × 10−3). The photoreactivity of acifluorfen was then studied in conditions approaching environmental conditions. Acifluorfen was dissolved in pure water, in water containing humic substances or in a natural water, and exposed to solar light in June at Clermont‐Ferrand (latitude 46 °N). In pure water, the half‐life was estimated at 10 days and photo‐decarboxylation accounted for 30% of the conversion. The presence of humic substances (10 mg litre−1) did not affect the rate of photo‐transformation. However, the half‐life of acifluorfen dissolved in the natural water was only 6.8 days.© 2001 Society of Chemical Industry