The photochemistry of the bichromophoric sulfonylurea chlorsulfuron has been investigated at different irradiation wavelengths and environmental conditions. The identified products indicate that different reaction paths are followed, depending whether the benzene or the triazine component chromophores are excited. In the first case, the chlorine atom is substituted by hydrogen or hydroxyl in water; in the second one three competitive photodegradation paths may be followed, the most efficient one being the cleavage of the S–N bond in the sulfonylurea bridge. Photodegradation quantum yield measurements, combined with laser flash photolysis experiments have shown that, while both singlet and triplet states contribute to all photolytic paths in deaerated solution, quenching by oxygen makes the triplet contribution negligible in samples equilibrated with air.