An extensive microcosm study was conducted to investigate the biodegradation of 1,2-dibromoethane (EDB) under in situ and biostimulated conditions within a plume at the Massachusetts Military Reservation in Cape Cod, MA. This particular EDB plume is unique because it has persisted for over 38 years, is more than 61 m below the ground surface, and has both aerobic and anaerobic zones with EDB levels above the maximum contaminant level (MCL) of 0.05 μg/L (ppb). Microcosms were constructed with in situ materials and conducted under environmentally relevant conditions (field EDB concentrations; incubated at 12°C). The results showed that natural attenuation occurred under anaerobic conditions but not under aerobic conditions. The lack of natural attenuation occurring in the aerobic zone, which is much larger than the anaerobic zone, offers valuable insight as to why EDB is so persistent at this site. EDB degradation rates were greater under biostimulated conditions for both the aerobic and anaerobic microcosms. On average, methane-amended aerobic microcosms degraded EDB at a first order rate eight times faster than unamended microcosms with the best performing replicate showing EDB degradation at a rate of 7.0 yr-1 (half-life (t1/2) = 0.10 yr). The lactate amended anaerobic microcosms degraded EDB at an average first order rate of 3.5 yr-1 (t1/2 = 0.20 yr) which was 19% faster than the unamended anaerobic microcosms. These results indicate potential for enhanced natural attenuation at the FS-12 site, especially under aerobic conditions.