In the present study, the effects of Fenton process on anaerobic sludge bioprocessing were investigated. A ratio of 0.067 g Fe(II) per gram H2O2, and 60 g H2O2/kg dried solids (DS) was applied to biological sludge samples preceding anaerobic sludge digestion. Single stage anaerobic digestion under thermophilic conditions is compared with two-stage anaerobic digestion (mesophilic digestion prior to thermophilic digestion). The comparison is in terms of solid reductions and specific methane productions. Fenton processed sludge gives higher solid reduction and higher methane production for each experiment. The highest reduction in sludge's solids was observed in a reactor operated under thermophilic conditions. The second stage digestion under mesophilic conditions did not induce extra solid reduction. However. it facilitated higher methane. Another observation is that, Fenton process led to decrease the biosolids' resistance to dewatering in terms of capillary suction time (CST), but had no effect on sludge dewatering on belt-press application. (c) 2009 Elsevier B.V. All rights reserved.