In this paper, the co-combustion behaviour of sewage sludge with straw and coal were carried out in a thermogravimetric analyzer under different atmospheres and at different heating rates (10, 20 and 40 K min(-1)) in the temperature range from ambient temperature to 1000 degrees C. TG and DTG curves were analysed. The Flynn-Wall-Ozawa's (FWO) and Kissinger-Akahira-Sunose's (KAS) isoconversional methods were used to yield dependency of the activation energy of reduction process on the degree of conversion. The values of E(alpha) were obtained. The results indicate that: with the increase of heating rate, the maximum weight loss rate of samples increase obviously. The activation energy is practically constant in the 0.2 <= alpha <= 0.9 range, with the average values of E(alpha) = 137.27 and 132.38 kJ mol(-1) calculated by FWO and KAS methods, respectively. (C) 2008 Elsevier Ltd. All rights reserved.