The thermal degradation of a series of para-substituted poly(styrene peroxide)s with electron-donating [CH3, C(CH3)3] and electron-attracting (Br) substitutents are investigated by thermogravimetric analysis (TGA). The results indicate that the Hammett relationship can describe quantitatively the trends in maximum rate of polymer decomposition (Tmax) observed in TGA and thus thermostability of substituted poly(styrene peroxide)s depends only on the electronic nature of substituents and their ability to stabilize macroradicals formed during chain scission. The experimental results are also substantiated by thermochemical calculations. © 2002 Elsevier Science Ltd. All rights reserved.