UV irradiation of metoxuron in aerated aqueoussolution at 254 nm or between 300 and 450 nm led initially to an almost specific photohydrolysis of the C-Cl bond, resulting in the formation of 3-(3-hydroxy-4-methoxyphenyl)-1,1-dimethylurea (MX3) and hydrogen chloride. The quantum yield was determined to be 0.020 (+/- 0.005) in solutions irradiated at 254 nm. Five minor photoproducts were also identified