Electron spin resonance studies on polystyrene (PS), polypropylene (PP) and on 80/20 PS/PP blends with and without compatibilizer (block SBS), at 7.5wt%, irradiated with gamma rays from a 60Co source at a dose rate of 4.8kGy/h and at integral irradiation doses of 10, 25, 50, 60, 70, 400, 800 and 1300kGy in the presence of air and at room temperature are reported. Dependence of resonance line width, Hpp, resonance line shapes, K, and resonance line intensity, Ipp, on the integral dose of irradiation is investigated. Nature of free radicals after 10 days of air storage is discussed.