Different activated sludges from pulp and paper mills were treated with hydrogen peroxide in the presence of ferrous sulphate under different reaction conditions in order to improve the dewatering characteristics of the sludges. The efficiency of the treatment was evaluated on the basis of the specific filtration resistance. Although sludges from different mills varied considerably with respect to their chemical composition and dewatering properties, the specific resistance values of all the sludges could be decreased using rather mild conditions. The filterabilities of the sludges were dependent on reaction temperature, hydrogen peroxide concentration and the reaction time used in the oxidative treatment. Depending on the type of sludge, hydrogen peroxide concentrations of 15 to 30 % mmoles/l sludge were sufficient to decrease the filtration values to a low level at 70°C using a reaction time of 1-2 hours at pH 3.