Previous experiments in our laboratory have demonstrated that the rate and extent of carbon tetrachloride (CT) and chloroform (CF) dechlorination were enhanced when a methanogenic enrichment culture and iron (Fe0) were incubated together. Batch experiments with three pure cultures of methanogens, Methanosarcina barkeri, Methanosarcina thermophila, and Methanosaeta concillii were performed to determine how this enhanced transformation occurred. When hydrogen (H2) was added as an electron donor, degradation of CT for all organisms and CF for M. thermophila was more rapid. H2 was produced from the oxidation of iron, which therefore served as an H2 source for the organisms, enhancing the transformation of CT and CF. Experiments with M. thermophila and M. concillii, which could not grow on H2-CO2 under the conditions tested, showed that H2 could serve as an electron donor for dechlorination of CT and CF with these organisms as well. Experiments with supernatants from M. thermophila grown with and without iron indicated the presence of an excreted biomolecule active in the enhanced transformation of CT and CF.