Oxygen uptake by isotactic polypropylene was studied at temperatures below melting point, on samples of various degrees of crystallinity and spherulite size. Diffusion coefficients and activation energy of diffusion were calculated. It is suggested that the dependence of the rate of oxygen uptake on the morphological structure of polymer samples can result from different values of the interspherulitic amorphous phase fraction, which is more easily accessible to oxygen than the interlamellar fraction.