AbstractCarbon and chlorine isotope effects for biotransformation of chloroform by different microbes show significant variability. Reductive dehalogenases (RDase) enzymes contain different cobamides, affecting substrate preferences, growth yields, and dechlorination rates and extent. We investigate the role of cobamide type on carbon and chlorine isotopic signals observed during reductive dechlorination of chloroform by the RDase CfrA. Microcosm experiments with two subcultures of a Dehalobacter‐containing culture expressing CfrA—one with exogenous cobamide (Vitamin B12, B12+) and one without (to drive native cobamide production)—resulted in a markedly smaller carbon isotope enrichment factor (εC, bulk) for B12− (−22.1 ± 1.9‰) compared to B12+ (−26.8 ± 3.2‰). Both cultures exhibited significant chlorine isotope fractionation, and although a lower εCl, bulk was observed for B12− (−6.17 ± 0.72‰) compared to B12+ (−6.86 ± 0.77‰) cultures, these values are not statistically different. Importantly, dual‐isotope plots produced identical slopes of ΛCl/C (ΛCl/C, B12+ = 3.41 ± 0.15, ΛCl/C, B12− = 3.39 ± 0.15), suggesting the same reaction mechanism is involved in both experiments, independent of the lower cobamide bases. A nonisotopically fractionating masking effect may explain the smaller fractionations observed for the B12− containing culture.