The effect of changes in redox potential on methanogenesis from acetate, and on the reductive dechlorination of pentachlorophenol (PCP), was evaluated using a computer-monitored and feedback-controlled bioreactor. PCP was transformed via 2,3,4,5-tetrachlorophenol (2,3,4,5-TeCP) to 3,4,5-trichlorophenol (3,4,5-TCP). In 6- to 12-d experiments, pH, acetate concentration, and temperature were held constant; the redox potential, defined here as the potential measured at a platinum electrode (E-Pt), was maintained at different set points, while transformation of multiple PCP additions was monitored. Without redox potential control, the value of E-Pt for the culture was approximately -0.26 V (vs. SHE). The value of E-Pt was elevated from -0.26 V for periods up to 10 h by computer-controlled addition of H2O2 or K3Fe(CN)(6). Methanogenesis continued during a relatively mild shift of E-Pt to -0.2 V with H2O2, but was halted when E-Pt was raised to -0.1 V with either H2O2 or K3Fe(CN)(6). Methanogenesis resumed when E-Pt returned to -0.26 V. During periods in which E-Pt was elevated significantly and methanogenesis stopped, transformation of PCP and 2,3,4,5-TeCP continued at progressively slower rates, but the rate of 2,3,4,5-TeCP transformation was diminished to a greater extent. When a small volume of pure H-2 was added to the reactor headspace, while E-Pt was maintained at -0.1 V, reductive dechlorination rates increased dramatically. Lower H-2 concentrations during periods of oxidant addition, perhaps due to the effect of the oxidant on H-2-producing bacteria, may contribute to decreased reductive dechlorination rates.