The results of comparative experiments on aerobic digestion of excessive activated sludge using hydrogen peroxide and Fenton's reagent were described. Fenton's reagent was found to have a higher oxidation potential and, as a consequence, higher efficiency in a digestion process in comparison with hydrogen peroxide. Moreover, Fenton's reagent was shown to improve sedimentation properties of sludge and to decrease soluble COD more efficiently than H2O2. The main advantage of the Fenton's reagent application was the fact, that oxidation processes took place even though Fenton's reagent was no longer added. On the other hand, the apparent disadvantage of its application is formation of additional chemical sediments and possible decomposition of sludge flocs as a result of overdosage of reagents and, consequently, increase in turbidity of supernatant liquid and some difficulties with sludge dewatering.