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The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were −7.2‰ and −8.5‰ for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only −0.3‰ for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of −25.2‰ for VC and −18.5‰ for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichment factors were larger than those of aerobic oxidation (−1.8‰ for VC, −1.5‰ for cDCE …