The influence of Fe-0/Fe2+/Fe3+ on the nitrobenzene (NB) degradation in the anaerobic granular sludge was studied and the results demonstrated that: adding iron powder into the anaerobic sludge could exert an accelerative effect on the NB degradation and the degradation rate was faster than that by using iron or the anaerobic sludge alone. The external addition of Fe2+/Fe3+ exhibited different influences on the NIB degradation depending on the concentrations of Fe2+/Fe3+ and the solution's pH. When Fe2+/Fe3+ are less than 100 mg/L at pH 6, Fe2+/Fe3+ inhibited the NB degradation slightly, and when Fe2+/Fe3+ was 100-200 mg/L, the NB degradation was enhanced. When pH was shifted to 9, Fe2+ of lower than 100 mg/L promoted the NB degradation, and 200 mg/L Fe2+ inhibit the NB degradation. The synergism of combined use of iron and the anaerobic sludge in treating NB wastewater was proposed, and adjusting the concentrations of Fe2+/Fe3+ in the anaerobic sludge according to the pH of the wastewater could be an effective method to obtain a high removal rate of NB. (C) 2006 Elsevier B.V. All rights reserved.