In the field of water treatment, one increased concern over the quality of the environment requires an understanding of the fate of compounds generated by the addition of chemicals. One area of considerable interest is the stability of chlorine compounds produced when chlorine is added to natural water or swimming pool water. It is desirable to be able to predict the lifetimes of these harmful compounds under various conditions. In this study we examine for a range of hypochlorite α-amino acid ratios and pH, the kinetics and mechanism of the decomposition of α(N-chloro) and α(N,N-dichloro) amino acid, one of the products of chlorination. The interaction of chlorine with amino acids has been studied by several investigators Langheld (1909) was the first who discussed the decomposition of the α(N-chloro) amino acids. He noted that hypochlorous acid salts react with α-amino acids in the same manner as they do with amines to form monochlorinated or dichlorinated derivatives. Then, the decomposition of chloro-amino acids leads to the corresponding aldehydes or ketones, ammonia, carbonic acid, and sodium chloride. As an intermediate step Langheld assumed an imine formation. Wright (1936) and Pereira et al. (1973) have investigated the decomposition products of α(N,N-dichloro) amino acids. Their results indicate rapid formation of carbon dioxide, chloride ion, and the corresponding nitrile. Recently, many authors have investigated the rates of α(N-chloro) amino acids decomposition and the stability of its products (William and Wendy, 1979; Yoshiro et al., 1980; Le Cloirec-Renaud, 1984). However, they have neither differentiated between the decomposition of α(N-chloro) amino acid and α(N,N-dichloro) amino acid, nor have they demonstrated the combined effect of pH and molar ratio of hypochlorite and α-amino acid. In this study the hypochlorite oxidation of simple α-amino acids in aqueous solution has been investigated in the dark. The concentration of α(N-chloro) amino acid and α(N,N-dichloro) amino acid was monitored by DPD-fast titrimetric method and by measuring the absorbance at 255 and 293 nm respectively, this is illustrated in Figs 3 and 4. These results and the amino acids determination (O-phtalaldehyde—2 mercapto ethanol method) suggest that the intermediates α(N-chloro) and α(N,N-dichloro) amino acid are formed rapidly at an initial stage. Then, they decompose spontaneously by first order kinetics as shown in Table 1, to give a mixture of aldehyde and nitrile. When equimolar (1:1 mmol) amounts of hypochlorite and amino acid are used at pH 7, only aldehyde, carbon dioxide, chloride and ammonia are formed. However the corresponding nitrile compound appears, when operating condition allow the formation of α(N,N-dichloro) amino acid (acid pH or basic aqueous solutions with high molar ratio of hypochlorite and amino acid). This is illustrated in Table 2. The rate constant shows a dependence on pH, which is caused by the various forms that can arise from addition of protons to or removal of protons from the amino and carboxyl groups of the molecule William and Wendy, 1979). We assume an intermediate step of imine for the decomposition of both compounds: α(N-chloro) and α(N,N-dichloro) amino acid (scheme 6). The reaction should be considered as a spontaneous decarbonylation followed by a rapid hydrolysis of the imine. Scheme 7 illustrated how α(N,N-dichloro) amino acid can lead to the corresponding nitrile and aldehyde, however the α(N-chloro) amino acid gives only the corresponding aldehyde. The products of decomposition of α(N-chloro) amino acid are relatively stable in aqueous solution. Although we noted at pH = 3.5–5 that aldehyde react with chloramines and lead to the formation of corresponding nitrile, as shown in scheme 9. It appears that α(N-chloro) and α(N,N-dichloro) amino acid formed during the chlorination of natural or swimming pool water will degrade in a few hours to what are probably irritating products (like as aldehydes). The production of decomposition are a function of molar ratio of hypochlorite and amino acid and pH. However, since most natural water has a pH in the range of 5.5–9, there will be little variation of the rate of decomposition with pH. It seems that it is only temperature dependent.