The fate and dechlorination kinetics of PCP and its intermediates were studied in two fluidized-bed reactors (FBRs) with methanogenic enrichments. The two FBRs were operated at a 1-day hydraulic retention time and fed lactate at 400 mg/L and pentachlorophenol (PCP) at 5 mg/L (18.8 μmol/L). PCP was meta-dechlorinated to 2,3,4,6-tetrachlorophenol (2346-TeCP). Approximately 75% of the 2346-TeCP was meta-dechlorinated to 2,4,6-trichlorophenol (246-TCP), followed by sequential o-chlorine removals to 4-monochlorophenol (4-MCP); the remaining 2346-TeCP was ortho-dechlorinated via 245-TCP to 3,4-dichlorophenol (34-DCP). Neither 34-DCP nor 4-MCP were dechlorinated throughout the 18-month testing period. In mixed-CP batch tests, CPs competed with each other, resulting in reduced dechlorination rates; competition was position-specific with respect to ortho- and meta-dechlorination reactions. Michaelis−Menten dechlorination kinetic coefficients and linear solid−liquid partition coefficients were determined for PCP, 2...