1,2-Dibromethane (EDB) is a toxic fuel additive that likely occurs at many sites where leaded fuels have impacted groundwater. This study quantified carbon (C) isotope fractionation of EDB associated with anaerobic and aerobic biodegradation, abiotic degradation by iron sulfides, and abiotic hydrolysis. These processes likely contribute to EDB degradation in source zones (biodegradation) and in more dilute plumes (hydrolysis). Mixed anaerobic cultures containing dehalogenating organisms (e.g., Dehaloccoides spp.) were examined, as were aerobic cultures that degrade EDB cometabolically. Bulk C isotope enrichment factors (εbulk) associated with biological degradation covered a large range, with mixed anaerobic cultures fractionating more (εbulk from -8 to -20‰) than aerobic cultures (εbulk from -3 to -6‰). εbulk magnitudes associated with the abiotic processes (dihaloelimination by FeS/FeS2 and hydrolysis) were large but fairly well constrained (εbulk from -19 to -29‰). As expected, oxidative mechanisms fractionated EDB less than dihaloelimination and substitution mechanisms, and biological systems exhibited a larger range of fractionation, potentially due to isotope masking effects. In addition to quantifying and discussing εbulk values, which are highly relevant for quantifying in situ EDB degradation, an innovative approach for constraining the age of EDB in the aqueous phase, based on fractionation during hydrolysis, is described.