The effect of thermal and sonication pre-treatment on the anaerobic degradation of sewage sludge was evaluated through the calculation of performance parameters by using three simplified mathematical models and one kinetic model. The Modified Gompertz equation, the Logistic function, Reaction Curve and First-Order models were all used with experimental data from the anaerobic biodegradability tests fed with primary and secondary thermal pre-treated sludge, and secondary sonicated sludge. All the models fit well with the experimental data, but the Reaction Curve model presented the best agreement in the fitting process. From the first-order equation no significant changes were observed in the hydrolysis constant under all conditions. Thermal pre-treatment (175 degrees C and 30 min) showed an important effect on the secondary sludge reaching an improvement of around 90% and 80% in the maximum production rate and the total biogas produced respectively. With regards to the sonication experiment, the best result was obtained when 12,400 kJ/kgTS were used, reaching an improvement of 40% in the total biogas production. (c) 2010 Elsevier B.V. All rights reserved.