In this study, possible toxicity of increasing doses of PCB-118 and transformer oil (TO) on anaerobic sludge digestion was investigated. For this purpose, five different sets of reactors were prepared in which four different PCB-118 concentration (1, 10, 20, and 30 mg L-1) and three different TO concentration (0.38, 0.76, and 1.52 g L-1) were applied. Throughout the study, biogas production and composition, pH, TS, VS, and COD as well as PCB concentration were monitored. Toxicity was investigated by anaerobic toxicity assay (ATA) evaluating the reduction in methane production. A notable inhibition was observed mostly in 30 mg L-1 PCB reactors. A negative influence of PCB-118 and TO was observed on COD and solids removal. A maximum of 26.5% PCB-118 removal was attained. (c) 2013 Elsevier Ltd. All rights reserved.