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. Both anaerobic bioreactors were operated under similar conditions during acclimation and initial loading of PCP. After stable performance was achieved, the operating conditions on the reactors were varied to study the effect of empty bed contact time (EBCT) under conditions of constant PCP loading rates with the optimum goal of minimizing the EBCT that permits for stable operation to occur.