Differential scanning calorimetry (DSC) technique was used to investigate the kinetics of crystallization in Se81.5Te16Sb2.5 chalcogenide glass. Non-isothermal measurements were performed at different heating rates (5-60K/min). A strong heating rate dependence of the activation energy was observed when the data were analyzed using Matusita model. This variation of the activation energy was confirmed by the application of the isoconversional methods of Friedman, Kissinger-Akahira-Sunose (KAS) and Vyazovkin. These methods showed that the activation energy of crystallization is not constant but varies with the degree of conversion and hence with temperature. However. the values of the activation energy obtained from Friedman method are much lower compared with the values estimated from KAS and Vyazovkin methods. it is suggested that this disagreement was due to possible errors involved in the Friedman method. (C) 2009 Elsevier B.V. All rights reserved.