Vinyl chloride (VC) is a carcinogenic contaminant commonly found in groundwater. Much research has focused on anaerobic reductive dechlorination of VC, and recently on aerobic VC degradation. In this study, the stable carbon isotope enrichment factor associated with aerobic VC assimilation was determined for Mycobacterium sp. strains JS60, JS61, and JS617 and Nocardioides sp. strain JS614. The enrichment factors ranged from -8.2+/-0.1 to -7.0+/-0.3 % and did not change as a function of biomass concentration. The measured enrichment factors for aerobic VC degradation were smaller than those reported for anaerobic VC degradation. Enrichment factors can also be expressed in terms of kinetic isotope effects (KIEs), 12k/13k, which result from the difference in reaction rates of bonds containing light and heavy isotopes. The KIEs for aerobic VC degradation (1.01+/-0.001) were smaller than those for anaerobic VC degradation (1.03+/-0.007). From the perspective of bond breakage during a chemical reaction, the larger KIE associated with anaerobic VC degradation as compared to aerobic VC degradation agrees with KIE theory. This theory predicts that larger fractionations can be expected in reactions where heavier atoms are involved (i.e., C-Cl bond for anaerobic versus C=C for aerobic) and in reactions involving large changes in vibrational frequencies of the molecule between its ground state and transition state (i.e., C-Cl cleavage versus C=C epoxidation). The significant fractionation observed during aerobic VC degradation suggests that stable carbon isotope measurements may be used as a tool to distinguish between biodegraded and nonbiodegraded VC.