A simple environmentally friendly one-step synthetic procedure was developed for S- and N-methylcarbamoylation of amino acids and their derivatives in buffered aqueous solutions. N-Succinimidyl N-methylcarbamate (SNMC) and N,S-dimethylthiocarbamate (DMTC) were used as synthetic equivalents to replace highly hazardous methyl isocyanate (MIC). SNMC reacted rapidly in both S- and N-methylcarbamoylations affording nearly quantitative conversions (>97 %) of all tested compounds after 2-3 h at 50 °C at pH 8.2 and 9.5-10 for S- and N-methylcarbamoylations, respectively. Under the same conditions, DMTC reacted more slowly (48 h for N-methylcarbamoylations) but with some polar amino acids it provided products of higher purity than SNMC. Similarly, N-trideuteriomethyl-S-methylthiocarbamate (DMTC-d 3 ) was used to synthesize N α - and N ε -trideuteriomethylcarbamoyl derivatives of valine and lysine, respectively. Prepared compounds will be used in toxicological research as authentic standards in the analyses for protein adducts derived from MIC or its metabolic precursor, N,N-dimethylformamide.