To gain some knowledge on the influence of a substrate morphology upon radiochemical grafting efficiency, the formation and the evolution of irradiation induced peroxy-radicals were measured by ESR in two polypropylene (PP) films of different origins. The creation rate of peroxy-radicals was found to be almost proportional to the irradiation dose in the film presenting the best crystallinity, the bi-oriented PP, while it saturates in the unoriented one. The recombination kinetics of the radicals appeared to be controlled by their diffusion in the matrix and the corresponding diffusion coefficients are dependent on the matrix itself and on its evolution under irradiation. It is suggested that the crystalline order of the matrix is the parameter governing the mobility and thus the lifetime of the radicals.