AbstractThe effect of oxygen on ESR spectra of various kinds of irradiated polymers has been examined by using a saturation technique developed in the present investigation. The radiation oxidation of some polymers has been studied by means of ESR as well as infrared spectroscopy. Results for polyethylene (PE) are given in this paper, with some general remarks as introduction. In PE three kinds of radicals are produced on irradiation: CH2CHCH2 (FRVI), CHCHCH (FRVII), and CH(CHCH)nCH2(FRI). On introduction of air, FRVIIgave the asymmetric singlet of the peroxide radical which showed anisotropy. Reversible oxygenation of FRVIItook place even as low as −113°C. Radiation oxidation of PE proceeds mainly via FRVII, which produced eventually about 12 carbonyl groups and about 5 hydroxyl groups in the course of post‐oxidation, thetrans‐vinylene group decreasing by an amount nearly equal to the amount of initial FRVII. FRVIplayed only a minor role in the oxidation. An oxidation mechanism involving chain reaction has been proposed. It suggests an intramolecular free‐back reaction of hydrogen to the peroxide radical of FRVII. FRI(polyenyl radicals of longer conjugations) is oxidized to form carbonyl and hydroxyl groups, although it does not give the spectrum of the peroxide radical. Polyenyl radicals