The aim of this study was to investigate the anaerobic digestion of waste activated sludge in the presence of a model PCB congener, 2,3',4,4',5-pentachlorobiphenyl (PCB-118), and transformer oil (TO), and thereby PCB dechlorination under mesophilic digestion conditions. Two PCB-118 concentrations (1 and 20 mg/L) and one TO concentration (1.52 g/L) were studied. Beside the PCB concentrations, pH, ORP, TS, VS, VSS, TSS, tCOD, and sCOD were monitored throughout the reactor operations (159-day). Methane productions of PCB reactors were considerably lower than that of control reactor (R-C), indicating the negative effect of PCB on methane production. The highest COD removal, 56%, was observed in R-C, followed by 1 and 20 mg/L PCB-118 reactors (46.7% and 40.6% reduction), respectively. VS reductions were between 36 and 52%, with the lower removal in the higher PCB-118 containing reactor. About 12 and 22% PCB-118 removals were attained proportional to their initial concentrations, 1 and 20 mg/L, respectively. (C) 2013 Elsevier Ltd. All rights reserved.