Different organic and inorganic chemicals including alum, ferric chloride, ferric sulphate, lime and polymers are used for conditioning. In this study, Fenton's reagent (Fe2+/H2O2), which has not been examined sufficiently so far, was used as a chemical conditioner. The effects of various Fe2+ (1000-6000mg l(-1)) and H2O2 (2000-6000 mg l(-1)) concentrations on specific resistance to filtration (SRF) and capillary suction time (CST) of biological sludge were investigated. Experimental results indicated that high Fe2+ and H2O2 concentration provides higher dewatering efficiency. Minimum SRF, 6.149 x 10(9) m kg(-1), was achieved at 5000 mg l(-1) Fe2+ and 6000 mg H2O2 concentration and minimum CST, 15.7 s, was obtained at dosage of 5000 mg l(-1) Fe2+ and 6000 mg l(-1) H2O2 (C) 2003 Elsevier Ltd. All rights reserved.