AbstractÐThe rate of photodegradation of the fungicide fenarimol (a-(2-chlorophenyl)-a-(4-chlorophenyl)-5-pyrimidine-methanol) by solar radiation has been studied in natural waters with dierent salinities. Photodegradation rates and quantum yields are found to decrease with increasing salinity.To explain this behaviour, experiments were performed with dierent salt solutions (NaCl, NaBr, BaCl 2 and ZnCl 2 ) using both solar irradiation and a photoreactor (l excit 313 nm). As with natural waters, marked eects of added salt were observed on the photolysis rate, which decreased signi®cantly from 0.67 h À1 in the absence of salt to 0.11 h À1 in the presence of BaCl 2 . In photoreactor studies, the observed order of quenching was BaCl 2 > ZnCl 2 and NaBr > NaCl. In contrast, neither the triplet state quencher sorbic acid nor oxygen had any eect on the photolysis, supporting the idea that photodegradation goes by the lowest excited singlet state of fenarimol.To understand the origin of the eect of added salts,¯uorescence quenching studies have been performed with various halide and nonhalide salts and the respective quenching constants determined. With NaCl and NaBr, the ratio of¯uorescence quenching constants is identical to the ratio of reciprocal quantum yields for photodegradation of fenarimol, indicating a common mechanism. The correlation of¯uorescence quenching rates with halide ion oxidation potentials strongly suggests that this involves an electron transfer mechanism. It is suggested that the eect may be used to stabilise the fungicide towards photodegradation. #