To elucidate the photochemical behavior of two sulfonylureas (cinosulfuron and triasulfuron) for which the chemical formulas are relatively close, their photodegradation was studied in water. All experiments were carried out under laboratory conditions using a xenon arc lamp as the source of radiation to simulate environmental conditions. Polychromatic quantum efficiencies were calculated to determine the photochemical pesticide lifetimes at pH 7, and a comparison with hydrolysis lifetimes has been performed. The results obtained showed clearly that at pH 7, photodegradation becomes a more important pathway than chemical degradation. HPLC-DAD was used to study the kinetics for both sulfonylureas and their photoproducts, whereas HPLC-MS (ESI in positive and negative modes) was used to identify photoproducts. These results suggest that the photodegradation of these two sulfonylureas proceeds via a number of reaction pathways: (1) cleavage of the sulfonylurea bridge; (2) desulfonylation, which can proceed either by a carbon-sulfur cleavage or a nitrogen-sulfur cleavage; (3) O-demethylation of methoxy moieties present on the triazine ring; and (4) O-dealkylation of benzene derivatives. In addition, it was found that the desulfonylation represented the main step and that it was wavelength dependent.