Abstract The influence of surfactants on a perchloroethene (PCE) dechlorinating mixed culture was investigated in laboratory experiments. Surfactants (Steol CS-330, Aerosol MA 80-I, alpha olefin sulfonate 14 to 16, Neodol 25-7, Tween 80, alkyl polyglycoside, C16TAB [trimethylammonium bromide], and sodium dodecyl sulfate) were evaluated for their effects on the rate and extent of PCE reductive dechlorination and their potential biodegradation by the mixed culture. Limited, if any, surfactant biodegradation was observed for the surfactants tested, and all surfactants impaired dechlorination in either the rate of PCE dechlorination or the terminal dechlorination products observed. Based on initial testing, a nonionic surfactant, Tween 80, and an anionic surfactant, Steol CS-330, were selected for additional investigation. Dechlorination of PCE to dichloroethene (DCE), vinyl chloride (VC), and ethene (ETH) occurred in all Tween 80–amended microcosms, with a depressed rate of ETH production as the only adverse effect. Steol CS-330, however, inhibited dechlorination beyond DCE at all surfactant concentrations exceeding 25 mg/L. Attempts to acclimate a culture to Steol CS-330 were unsuccessful. Inhibition of VC and ETH production was reversible on dilution of the surfactant to a concentration of 10 mg/L or less, indicating that surfactant interactions with the enzyme system responsible for reductive dechlorination of DCE may be the cause of inhibition.