Photochemical degradation of acetochlor--2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methyl-phenyl)- acetamid--was studied at ambient temperature. The herbicide was irradiated by UV-light in methanol and water-methanol solvent mixture. It was found that the photodecomposition of acetochlor obeys first order kinetics under the condition of photolysis. The half-life of the reactant was found as 1.2 h in water-methanol solvent mixture, while just half of that could be observed in pure methanol. The first step of the photolysis is the dehalogenation which is indicated by fast increasing of chloride concentration in the solution, and simultaneously an equivalent amount of protons is forming, too. The aromatic ring seems to be intact but the substituents connected to the nitrogen can be changed in many different ways. Among the photo-products 10 metabolites were identified by GC-MS method, they showed higher photochemical stability than the parent herbicide.