Research was conducted, which aim was to evaluate quantitative and qualitative PCB changes in sewage sludge during mesophilic digestion, and the influence of those compound concentration above acceptable level (1.9 mg kg(-1) of sludge dry matter), also nickel and cadmium ions, on mesophilic digestion process. Before and after the digestion, concentration of seven PCB congeners, Ni and Cd in particular chemical fractions of the sludge was analyzed. During mesophilic digestion it was stated that increased PCB and heavy metal ions content in sewage sludge, did not disturb correct progress of the process. Last days of digestion were important for intensive degradation of higher chlorinated PCB to congeners containing less chlorine atoms in particle. In liquid of sludge with increased PCB content, up until the 14th day of mesophilic digestion process, gradual increase in amount and sorts of indicator PCB congeners was observed. First LCB were released into sludge liquid. Total amount of PCB in sludge after digestion was decreased significantly, which indicate that biological degradation of those compounds under anaerobic conditions, and/or biosorption of those compounds, proceeded. It has been proved that the digestion of sludge excessively loaded with nickel ions can lead to an adverse accumulation of this metal in the exchangeable and carbonate fraction of stabilized sludge. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.