Floc properties and dewatering of activated sludge from the pulp and paper industry were studied prior to and after an oxidative conditioning using Fenton's reagent Sludge samples were taken from four treatment plants, which differed with respect to organic loadings and wastewater source. Poor dewaterability was typical of heavily loaded sludges and a positive correlation between filtrability and the concentration of exopolysaccharides was observed. Oxidative conditioning improved the dewaterability of all the sludges tested, but especially that of heavily loaded sludges. Lab-scale piston press results were also comparable with those of controls conditioned with polyelectrolytes. As a result of the conditioning, the surface charge density of the flocs decreased and the contact angles of filter cakes were increased, which enhanced flocculation and settleability. It is possible that hydroxyl groups are oxidized during the oxidative treatment to carboxyl groups, which are more hydrophobic at low pH. This hypothesis was partly supported by FTIR-analysis, but obviously other mechanisms were also involved.