This article describes an HPLC method for routine samples analysis of laboratory scale anaerobic immobilized sludge reactors used for degradation of pentachlorophenol (PCP) in synthetic wastewater. The method was developed to quantify pentachlorophenol and six chlorophenol isomers produced during the degradation of pentachlorophenol by anaerobic bacteria. The validated method was used for anaerobic reactor samples and was able to quantify the chlorophenol intermediates produced.