A critical analysis of the isoconversional methods for evaluating kinetic parameters of decomposition of solids from isothermal thermogravimetric data is presented. An isoconversional integral method to evaluate the activation energy is suggested. This method allows removing the errors due the correction of the degradation time by subtracting the induction period to onset of the main reaction (also including the time required to heat the sample to the temperature at which the isotherm is recorded). This procedure was used to study the degradation of two series of polymeric materials, (a) poly(vinyl chloride)/acrilonitrile-butadiene-styrene (PVC/ABS) blends and (b) poly(vinyl chloride)/chlorinated poly(ethylene) (PVC/CPE) blends. The values obtained for the activation energy are in fairly good agreement with those obtained from the Prout-Tompkins model in case (a) and from nonisothermal data in case (b). (C) 2003 Wiley Periodicals, Inc.