A kinetic model for the radiation induced oxidation of isotactic polypropylene film (38μ thick) has been developed which allows determination of the concentration profiles, rate constants and diffusion coefficients. The match with the experimental data is consistent with a distribution of reactivity that changes during the course of the reaction giving rise to time dependent fat? constants according to the Plonka's equation (k=ko(a-1). This equation can be rationalized within the framework of the random walk models under the assumption of temporal, geometric or energetic disorder. The origin and the characteristics of the reactivity distribution have been investigated by an ESR spectroscopy study of the degree of order and the molecular dynamics of the intermediate peroxy radicals.