:The major photoproducts observed in the photolysis of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] (2) in aqueous solution resulted from a heterolytic substitution of chlorine by OH (photohydrolysis). A wavelength e †ect was observed : at 254 nm the formation of 3-(4-chloro-3-hydroxyphenyl)-1,1-dimethylurea (3) accounted for more than 90% of the conversion, whereas when the solution was irradiated in "black lightÏ (85% of photons emitted at 365 nm, about 7% at 334 nm), the major photoproduct was 3-(3-chloro-4-hydroxyphenyl) -1,1-dimethylurea (4). The presence of methanol favoured the photoreduction into 3-(3-chlorophenyl)-1,1-dimethylurea (5).Completely di †erent reactions were observed when 2 was irradiated in dry aerobic conditions on silica. They resulted from elimination or oxidation of methyl groups. The main photoproducts initially formed were 3-(3,4-dichlorophenyl)-1-methyl urea (6) and 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea (7). In the second stage (6) was transformed into (3,4-dichlorophenyl)-urea (8) and 3-(3,4-dichlorophenyl)-1-formylurea (9) ; some other minor products such as monuron (1) were also identiÐed. The formation rate of 6 and 7 was much slower on clay (montmorillonite or kaolin) than on silica.In contrast with products 6 and 8, the formation of 7 and 9 needed the presence of oxygen : they did not appear when diuron was irradiated in deoxygenated It can be concluded that the photolysis of diuron is highly dependent C 2 Cl 3 F 3 . on the conditions of irradiation.

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:The main photoproducts initially formed in the phototransformation of linuron and chlorbromuron in aqueous solution result from photohydrolysis, i.e. hydroxylation with release of halide ion, and from elimination of a methoxy group. Reductive debromination was also observed with chlorbromuron. The transformation is less speciÐc than with diuron. The orientation of the reaction depends on the wavelength : short wavelengths (254 nm) favour demethoxylation and photohydrolysis in the meta position whereas, with "black lightÏ, i.e. wavelengths longer than 330 nm, photohydrolysis in the para position is the main reaction observed.

The major photoproducts observed in the photolysis of diuron [3-(3,4dichlorophenyl)-1,1-dimethylurea] (2) in aqueous solution resulted from a heterolytic substitution of chlorine by OH (photohydrolysis). A wavelength e †ect was observed : at 254 nm the formation of 3-(4-chloro-3-hydroxyphenyl)-1,1dimethylurea (3) accounted for more than 90% of the conversion, whereas when the solution was irradiated in "black lightÏ (85% of photons emitted at 365 nm, about 7% at 334 nm), the major photoproduct was 3-(3-chloro-4-hydroxyphenyl) -1,1-dimethylurea (4). The presence of methanol favoured the photoreduction into 3-(3-chlorophenyl)-1,1-dimethylurea (5).Completely di †erent reactions were observed when 2 was irradiated in dry aerobic conditions on silica. They resulted from elimination or oxidation of methyl groups. The main photoproducts initially formed were 3-(3,4-dichlorophenyl)-1-methyl urea (6) and 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea (7). In the second stage (6) was transformed into (3,4-dichlorophenyl)-urea (8) and 3-(3,4-dichlorophenyl)-1-formylurea (9) ; some other minor products such as monuron (1) were also identiÐed. The formation rate of 6 and 7 was much slower on clay (montmorillonite or kaolin) than on silica.In contrast with products 6 and 8, the formation of 7 and 9 needed the presence of oxygen : they did not appear when diuron was irradiated in deoxygenated It can be concluded that the photolysis of diuron is highly dependent C 2 Cl 3 F 3 . on the conditions of irradiation.