The most commonly used chloroacetamide herbicide, alachlor, and its conjugated adducts have been characterized by electrospray ionization mass spectrometry (ESI-MS). The reactivity of glutathione toward alachlor has been evaluated by changing experimental parameters, such as pH, temperature, and tube lens offset voltage (TLOV) in aqueous methanol, and the products were subjected to collision-induced dissociation (CID) for further characterization. In the positive mode, CID proves the formation of cyclic species by elimination of glycine and NH(3) moiety, which is similar to protonated cysteine. The results confirm that, only under basic conditions, glutathione is able to detoxify alachlor.