The ability of dehalogenating bacteria to compete with sulfate reducing bacteria for electron donor was studied in microcosms that simulated groundwater contaminated with both chlorinated ethylenes and fuel hydrocarbon compounds. Results demonstrate that reductive dehalogenation of perchloroethylene to ethylene can proceed in the presence of > 100 mg l(-1) sulfate. The hydrogen concentration, which was 2.5 nM in the presence of approximately 150 mg l(-1) sulfate and in the absence of chlorinated compounds, decreased to 0.7 nM during the dechlorination of trichloroethylene and increased to 1.6 nM during the dechlorination of cis-dichloroethylene and vinyl chloride. With only sediment associated donor ("historical" donor) present, dechlorination of trichloroethylene proceeded slowly to ethylene (on a time scale of several years). Addition of toluene, a model hydrocarbon compound, stimulated dechlorination indirectly. Toluene degradation was rapid and linked to sulfate utilization, and presumably formed fermentable substrates that served as hydrogen donors. Dehalogenation was inhibited in soil free microcosms containing 5 mM sulfide, but inhibition was not observed when either aquifer sediment or 5 mM ferrous chloride was added.