Soil microcosms were used to evaluate the potential benefits of an integrated microbial-Fe0 system to treat groundwater contamination by RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). Microcosms amended with both Fe0 filings and municipal anaerobic sludge mineralized RDX faster and to a greater extent than separate treatments, with up to 51% 14CO2 recovery after 77 d. The nitroso byproducts 1,3-dinitro-5-nitroso-1,3,5-triazacyclohexane (MNX), 1,3-dinitroso-5-nitro-1,3,5-triazacyclohexane (DNX), and 1,3,5-trinitroso-1,3,5-triazacyclohexane (TNX) were detected in all microcosms, although these compounds never accumulated above 5% of the added RDX on a molar basis. A soluble intermediate that was tentatively identified as methylenedinitramine [(O2NNH)2CH2] was relatively persistent, although it accumulated to a much lower extent in combined-treatment reactors than in sets with Fe0 or sludge alone. Some of the radiolabel was bound to soil and Fe0 and could not be extracted with CH3CN. This fraction, which was recovered by combustion with a biological oxidizer, was also found at lower concentrations in combined-treatment reactors. This work suggests that permeable reactive Fe0 barriers might be an effective approach to intercept and degrade RDX plumes and that treatment efficiency might be enhanced by biogeochemical interactions through bioaugmentation.