AbstractResults of ESR and infrared studies of radiation oxidation of polypropylene (PP) and poly(vinyl chloride) (PVC) are reported. The effect of oxygen on the free radicals, FRVIII(CH2Ċ(CH3)CH2) and FRIX(CH2Ċ(CH3)2), FRI′(polyenyl radicals with shorter conjugations), FRI(polyenyl radicals with longer conjugations) in PP and FRI′, FRIin PVC was studied. On introduction of air, FRVIIIand FRIXin PP and FRI′in PP and PVC showed the asymmetric singlet spectrum of the peroxide radical. Oxygenation of the radicals began at low temperatures: −135°C. for FRVIIIand FRIX, −144°C. for FRp in PP, and −170°C. for FRI′in PVC. By reevacuation, the oxygen molecule could be removed from the peroxide radical of FRI′in PP and PVC, while deoxygenation was impossible from FRVIIIand FRIX. Oxidation of the radicals in PP and PVC gave carbonyl and hydroxyl groups. The number of carbonyl groups produced per radical was estimated as 3.4 and 1.6 for