The effect of double-walled carbon nanotubes (DWNT) on a trichloroethene (TCE)-dechlorinating culture was investigated. The culture dechlorinates TCE to dichloroethene (DCE), vinyl chloride (VC) and ethene, ferments butyrate (the hydrogen donor), and produces methane. We hypothesized that TCE would sorb to DWNT and be unavailable to the dechlorinating bacteria. DWNT, used as obtained from the vendor, were added to culture medium at 0, 27, 133, 400, 800, or 1600 mg/L, amended with TCE, and allowed to equilibrate. The DWNT solutions were inoculated and amended with butyrate. Production of dechlorination products and methane was monitored for 135 days. TCE was dechlorinated by the mixed culture up to the maximum loading of 1.6 g DWNT/L. The rate of dechlorination first increased as the DWNT loading was increased, and then decreased as DWNT loading increased beyond 800 mg/L. Methanogenesis decreased with DWNT loading increase. No methane was produced in cultures with 1600 mg DWNT/L.