A new model structure for the hydrolysis step is introduced in the IWA anaerobic digestion model no 1 (ADM1) in order to better represent the bioaccessibility of particulate organic matter. Two particulate organic matter fractions for waste activated sludge (WAS) samples were defined; a readily hydrolysable fraction (X-cr) and a slowly hydrolysable fraction (X-cs). These fractions were hydrolyzed according to a surface-limiting reaction. Batch anaerobic digestion test of untreated WAS was used to develop the model and calibrate the kinetic parameters and biomass concentrations. The validation was carried out with a similar substrate than the calibration but a thermal pretreatment was applied at two different conditions (110 degrees C and 220 degrees C). The behavior of thermophilic anaerobic digestion of WAS samples was effectively represented by the proposed model. No changes among kinetic parameter sets were done and the model is able to represent produced methane volume following the intrinsic changes of the WAS composition through the different thermal pretreatment conditions. Moreover, estimated parameters in 20 days of batch anaerobic digestion test were representative of continuous anaerobic digestion in a full-scale digester.