The study evaluates the use of two anaerobic fluidized-bed granular activated carbon (GAC) bioreactors for treating pentachlorophenol (PCP), one of the major toxic compounds found in wastewaters stemming primarily from wood preserving industries. The reactors are fed synthetic solutions of PCP in 95% ethanol, where ethanol is the primary substrate and PCP the cometabolite. A carbonate buffer and another solution containing growth nutrients and vitamins are also fed to the bioreactors. COD balances show that most of the effluent COD occurs in the effluent gas, containing 87-94% methane, indicating sustained anaerobic activity in the bioreactors. In addition, analysis of effluent PCP< mono-, di-, and trichlorophenols indicate that PCP is dechlorinated in the reactor to an equivalent molar concentration of 3- and 4-chlorophenol. (Copyright (c) 1993 Water Environment Federation.)