ESR spectroscopy has been used to follow the kinetics of decay of radicals produced by UV-radiation in PMMA films upon exposure to oxygen in the temperature range 160-210 K. In the same films and at the same conditions, decay kinetics of phenanthrene phosphorescence has been studied at 180-220 K. Both types of the experiments give the same values for diffusion coefficients of oxygen in PMMA. Thus, from the oxygen diffusion standpoint, the sites of the radical stabilization and phenanthrene molecule localization in the polymer matrix do not differ.