The treatment of wastewater generates a by-product, sewage sludge, the disposal of which poses problems. There are, however, a number of options for making use of this residue, including its conversion into a valuable fuel. The aim of this paper is to describe the process of the heating of sewage sludge under different atmospheres of helium and oxygen and to obtain the kinetic parameters by means of a mathematical model, a heating rate of 10degreesC/min being used. Measurements were taken by thermogravimetry and mass spectrometry, and proximate and elementary analyses were made of the sludges and their calorific values were determined. Visual observation of the heating profiles shows four stages in the heating process, which have been characterized. The model proposed was found to adequately describe the weight loss of the sludges studied, while making it possible to obtain the kinetic parameters for the differentiated stages in the thermal process but not to establish any clear tendency of the evolution of these parameters with the increase in oxygen in the heating atmosphere. (C) 2003 Elsevier B.V. All rights reserved.