Methyl methacrylate can be readily graft copolymerized to polypropylene film by mutual γ-irradiation in vacuo. Graft yields in excess of 1000% can be obtained at low total doses (0.1 Mrad) and dose rates (0.05 Mrad/hr). When the monomer is diluted with solvent mixtures, similar yields can be obtained at lower monomer concentrations due to an accelerative effect. Besides swelling of the polypropylene, the results indicate the importance of solvent on the reaction kinetics of the growing poly(methy1 methacrylate) radicals. A possible explanation of the effect of diluent is presented in terms of polymer chain entanglement.