The effects co-disposal of industrial sludge with the organic fraction of municipal solid wastes (OFMSW) on the anaerobic treatment of domestic solid waste and the leachate characteristics were investigated in three simulated landfilling anaerobic bioreactors. All of the reactors were operated with leachate recirculation. One of these was loaded only with OFMSW (control reactor); the second reactor was loaded with industrial sludge and OFMSW, the weight ratio of the OFMSW to industrial sludge was 1:1 (dry solid basis) (run 1); the third reactor was loaded with industrial sludge and OFMSW, the weight ratio of the OFMSW to industrial sludge was 1:2 (dry solid basis) (run 2) in order to compare the effects of co-disposal of industrial sludge with the solid wastes. The leachate recirculation rate was 300 ml/day in all of the reactors. pH, chemical oxygen demand (COD), volatile fatty acids (VFA), ammonium-nitrogen (NH4-N) concentrations in leachate samples; total and methane gas productions in simulated anaerobic reactors were regularly monitored. After 98 days of anaerobic incubation, it was observed that the pH, COD, VFA concentrations in the leachate samples of the industrial sludge-added reactors (especially run 2) were better than in the control reactor. The COD values were measured as 7128, 5714 and 4205 mg/l while the VFA concentrations were 1605, 13 10 and 576 mg/l, respectively, in the leachate samples of the control, runs 1 and 2 reactors after 98 days of anaerobic incubation. Although the control has a high initial BOD5 concentration, the highest BOD5 decrease occurred in this reactor. The addition of industrial sludge did not significantly retard the decreases in BOD5 concentrations. The TN, TP and NH4-N reduction in the control reactor are the highest level. The TN, TP and NH4 N concentrations in MSW reduced to 614, 314 and 121 from 10 900, 2300 and 500 mg/kg in control reactor by day 98. The values of pH were 6.31, 7.16 and 7.25, respectively, after anaerobic incubation, respectively in the control, runs I and 2 reactors. Industrial sludge addition decreased the methane percentage in the anaerobic simulated reactor. The methane percentage of the control, runs land 2 reactors were 31%, 20% and 12%, respectively, after 98 days of incubation. In addition, at the end of the anaerobic toxicity assay (ATA) test, there is significant toxicity in industrial sludge-added reactors (especially run 2) under anaerobic conditions since decreases in methane gas productions was observed. (c) 2004 Elsevier Ltd. All rights reserved.