Thermal degradation of phthalazinone-based poly(aryl ether sulfone 1,3,5-triazine) copolymers (PPESTs) has been investigated under nitrogen flow in dynamic heating conditions. Model-free kinetic approaches were applied to data for decomposition processes of PPESTs with different molecular structure. The activation energy was calculated as a function of the degradation extent by using Kissinger-Akahira-Sunose and Vyazovkin method. The effect of polymer composition and structure on the characteristic temperatures and kinetic parameters of thermal degradation was also investigated. The replacement of sulfone by 1,3,5-triazine in polymer main chain significantly increases initial decomposition temperature and Ea, and its stabilizing effect may be explained by the existence of two-stage decomposition process that suppresses unzipping of the copolymers. This assumption is supported by the experimental fact that the complete replacement of sulfone by 1,3,5-triazine increases the initial decomposition temperature by 31 degrees C and E, by at least 57 kJ/mol according to Vyazovkin method. (C) 2010 Elsevier B.V. All rights reserved.