The photochemistry of the herbicide chlorsulfuronR (2-chloro-N-[[4-methoxy-6-methyl-1.3.5-triazinyl-2-amino]carbonyl]benzene sulfonamide) was investigated in methanol, distilled water, natural creek-water, on silica gel and on montmorillonit surfaces. The rate of degradation was determined in outdoor and indoor experiments simulating tropospheric conditions (λ>290 nm). Two major photoproducts were identified in all cases.