Electron spin resonance (ESR) spectroscopy has been used for the measurement of the diffusion coefficient of oxygen into polymeric spheres. It has been shown that there is an inverse dependency between the irradiation dose and the oxygen diffusion coefficient (D). The free-radical concentration (R-0) produced in the polymer depends on the dose of the gamma rays. In this study, we tried to find a relationship between the oxygen diffusion coefficient and the free-radical concentration produced in the polymer during the irradiation.