Carbon stable isotope trichloroethylene (C-13 TCE) was used to investigate the formation of chloromethane (CM) during the electrolytic dechlorination of trichloroethylene (TCE) at a granular-graphite packed cathode. A method was developed to use a conventional GC/MS to analyze and quantify regular and C-13 TCE and their dechlorination products. The concentration of a C-13 compound can be calculated, based on the concentration of its regular counterpart, from the response ratio of two fragments of different mass per charge values from the compounds in a sample and two characteristic MS spectrum ratios: one is the response ratio of the two fragments of the regular compound, and the other is the response ratio of the corresponding fragments of the regular and C-13 compounds at the same concentrations. The method was used to analyze the regular and C-13 compounds observed in an experiment of dechlorination in an ammonium acetate solution that contained both regular TCE and C-13 TCE. Results of analysis confirmed that CM was not a direct product of TCE dechlorination at the granular graphite cathode that cis-DCE was an intermediate product of TCE dechlorination, and that 1,1-DCE was not a dechlorination product. Published by Elsevier B.V.