In order to face excess waste activated sludge management problems, sludge anaerobic digestion with thermal pre-treatment is of great interest. If most of works agree on the optimal treatment temperature (160-180 degrees C), results of thermal pre-treatments in terms of biogas production are very dispersed. With the aim of analysing the impact of sludge samples, thermal pre-treatments were carried on five different waste activated sludge samples. For temperatures lower than 200 degrees C, COD solubilisation was found to increase linearly with treatment temperature and all the different sludge samples behaved in the same way. For temperatures lower than 150 degrees C, carbohydrates solubilisation was more important than proteins solubilisation. Analyses of sludge apparent viscosity, settleabilty and dewaterability (CST) of pre-treated sludge pointed out a threshold temperature of 150 degrees C. Thermal treatments up to 190 degrees C allowed the biogas production to increase during batch anaerobic digestion of sludge. Biogas volume enhancement was linked to sludge COD solubilisation and to untreated sludge initial biodegradability: the lower the initial biodegradability, the higher is the impact of thermal treatment. (C) 2007 Elsevier B.V. All rights reserved.