Soil cores were collected within and adjacent to a trichloroethylen plume that has contaminated the shallow aquifer at Picatinny Arsenal, New Jersey, to study the rate of cis-1,2-dichloroethylene biotransformation. Soil cores also were collected at a highly contaminated site within the plume to determine the effect of different electron donors on the rate of cis-1,2-dichloroetylene biotransformation under methanogenic conditions. Ground-water samples were analyzed for volatile organic compounds to study the relation between volatile-organic-compound concentrations and the biotransformation of cis-1,2-dichloroethylene. Biotransformation in microcosms from sites within the plume ranged from slight to more than 90 percent after an incubation period of 32 weeks. The most extensive biotransformation in soil microcosms occurred at a site near the highest in situ cis-1,2-dichloroethylene concentration measured at the arsenal (710 micrograms per liter). Biotransformation was negligible at an uncontaminated site. Amendment of soil microcosms with combinations of methanol, formate, toluene, p-cresol, propionate, and butyrate inhibited the biotransformation of cis-1,2-dichloroethylene. A combination of methanol and formate had the greatest inhibitory effect.