Alachlor was incubated with hypochlorous acid for 24 hours. The herbicide and its transformation products were extracted by liquid-liquid extractions and analyzed by gas chromatography/ mass spectrometry. Six compounds were detected in the residue of the reaction between Alachlor and HOCl after 24 hr. of incubation at room temperature, whose spectral data indicated that they were derived from Alachlor via chlorination and hydrolysis reactions. Among the structural assignments, two products were confirmed by analysis of standards of which they were synthesized. The Alachlor was not detected in the reaction residue. In quantitative studies, at all of the concentrations of HOCl, the percentage remaining of the parent compound at pH 3.0 was significantly lower compared to the reaction at pH 7.0.the major product of Alachlor and ClO2 is N-(methoxycarbonyl)-N-(2,6-diethylphenyl) oxamic acid. The data showed that chlorination and cleavage of the N-methoxymethyl (MOM) group are significant features of the environmental degradation of Alachlor during water treatment with Cl2.