Environmental soil contamination at an industrial site in Marion, Ohio (USA) with tetrachloroethene (perchloroethene, PCE) resulted in residual cis-l, 2-dichloroethene (DCE) contamination that had not declined after more than 15 years. Microcosm slurries containing 2.6% soil from this site were supplemented with different electron donors, i.e., individual fatty acids or alcohols. None of the microcosms supported complete DCE dechlorination, unless PCE was added to the microcosm at initiation. The addition of fresh PCE resulted in the dehalogenation of PCE to DCE in the microcosms supplemented with fatty acids having an even number of carbon atoms (acetate, butyrate, and caproate), but not in those with an odd number of carbon atoms (formate, propionate, and valerate), where negligible or no activity was detected. No significant further DCE degradation was observed in any of the microcosms supplied with fatty acids as electron donors. Microcosms supplemented with freshly added PCE bioconverted PCE to DCE and completely dehalogenated both the ex-novo and soil-supplied DCE within 60 days, but only if alcohols having an even number of carbon atoms (ethanol or butanol) were also added as electron donors. Odd-numbered alcohols either did not produce dehalogenation (as with methanol) or only dehalogenated PCE to DCE (as with propanol).