Phenolphthalein-modified polyarylene ether nitrile copolymers were synthesized by nucleophilic substitution reaction of 2,6-difluorobenzonitrile (DFBN) with varying molar ratios of phenolphthalein (PP) and hydroquinone (HQ). The copolymers were chemically and physically characterized to have good thermal and mechanical properties. PP content of the copolymers has influence on glass transition temperature (Tg) as well as decomposition temperature (Tid). Coats–Redfern method was applied to study the thermal decomposition kinetics of the copolymers to predict the lifespan under thermal aging process of the copolymers, the results show that the lifespan increases with increasing PP content. © 2005 Elsevier B.V. All rights reserved.