Occurrence and production of stable chloramines in the chlorination of creatinine, a constituent of perspiration and urine, in aqueous media were studied. Creatinine (5 x 10(-5) M) was treated with free chlorine in aqueous solutions at molar ratios of 0.5-8 (chlorine/creatinine) at pH 7.0 at room temperature for several days. At lower ratios of chlorine, two stable N-chlorocreatinine derivatives, which were determined as dichloramine fractions by the DPD method, were isolated by HPLC and identified by EI-MS and H-1-NMR. One was 2-chloroamino-1-methylimidazolin-4-one (creatinine chloramine) and the other was 2-chloroamino-5-hydroxy-1-methylimidazolin-4-one (hydroxycreatinine chloramine). In addition, the formation of methylamine was identified by GC-MS analyses of its imine derivative formed with pentafluorobenzaldehyde. Methylamine forms stable chloramines, which might be determined as mono- and/or di-chloramine fractions together with free chlorine by the DPD method in the reaction mixtures at higher molar ratios of chlorine. In practice, small amounts of methylamine (ca. 19 mug/L) were detected in water samples collected from several swimming pools. Hence, methylamine may be an origin of elusive organic chloramine formed in the chlorination of swimming pools. A probable mechanism of the occurrence and processing of chlorination products of creatinine is suggested. (C) 2004 Elsevier Ltd. All rights reserved.