Kinetic modelling of the hydrolysis stage of municipal activated sludge, which is presumed to be the rate-limiting step in the anaerobic sludge digestion process, was studied by measuring methane production rate (MPR) in anaerobic batch tests. The MPR curves revealed that the degradable organic components in municipal sludge could be classified into two fractions having different kinetics. The first fraction (XS1) constituted about 55% of the sludge COD and degraded with first-order kinetics. The second fraction (XS2), which degraded during the initial phase, accounted for about 21% of sludge COD. The degradation kinetics for XS2 was expressed by Contois-type equation with respect to concentration of substrate in the fed sludge and that of active biomass in the mixture. Simultaneous batch aerobic respirometric tests showed that the activated sludge was composed of 53% heterotrophic biomass (XH-Aerobe) COD and 20% of slowly biodegradable COD (XS), that had same kinetic expressions as observed in the batch anaerobic tests. The observed correlation between substrate fractions suggests XS1 and XS2 could be directly mapped to the aerobic state variables of XH-Aerobe and XS respectively. The degradation of XS1 seems to be anaerobic decay of XH-Aerobe while XS2 is thought to be hydrolysis of XS by microcosm of the sludge.