The lactam of N,S-dicarboxymethylcysteine is usually not present in batch samples. The influence of the solutions pH on the formation of the lactam of N,S-dicarboxymethylcysteine were investigated by an HPLC-UV method and nuclear magnetic resonance (NMR)spectroscopy. The cyclization of N,S-dicarboxymethylcysteine was monitored by the appearance of the additional signals for the N CH2COOH group resonating at ı = 4.51 and 3.22 ppm. The structure is supported by the peak of m/z 218 in the mass spectrum.In conclusion, carbocisteine and N,S-dicarboxymethylcysteine are rapidly cyclized in acidic solution (the peak area of the lactam dou-bles within 30 min) and more stable in slightly basic solutions (seeFig. 5).