The effect of chemical modification on free radical behaviour was investigated employing electron spin resonance in a pair of amorphous polymers, namely tetramethyl bisphenol A polycarbonate (TMPC) and bisphenol A poly- carbonate (PC). The initial concentrations of free radicals, decay curves and decay kinetic characteristics are discussed using both free-volume data from our structural study by positron annihilation spectroscopy and dynamic data from dynamic-mechanical relaxation investigations in the literature. It was found that the initial concentrations of free radicals depend predominantly on free-volume structure, being higher for TMPC compared with PC due to a weaker cage effect. The extent of free-radical decay as well as the macroradical decay kinetic charac- teristics have been found to depend more on the internal dynamics than on dif- ferent physical structures, in contradiction to our previous investigation of the polymer with the same chemical structure (PC), but different physical structures introduced by different mechanical and/or thermal treatment.