Chemical modification of isotactic polypropylene via a free-radical mechanism was studied using an on-line electron spin resonance spectrometer. Peroxides were used to provide primary radicals upon thermal decomposition at elevated temperatures for the generation of polymer backbone radicals. Four types of peroxides, tert-butyl perbenzoate, dicumyl peroxide, 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexyne-3, and benzoyl peroxide, at various concentration levels were employed. The ESR measurement demonstrated that the modification was accomplished via a free-radical mechanism. Both radical generation and termination were clearly observed. Peroxide type, concentration, temperature, and reaction time were the major factors that affected the radical concentration profiles. The radical concentration variations were used to estimate the radical termination rate constants.