Abstract The graft copolymerization of 4-vinylpyridine onto LDPE films pre-irradiated with a 1.2 MeV electron beam, was carried out in the vapor phase as the second step in the preparation of charge-mosaic membranes. The effects of pre-irradiation dosage, atmosphere, storage time and storage temperature of PE films on the grafting process were studied by measuring the rate of free radical decay under different conditions. The results of ESR study show that: (1) The free radicals, created in the course of pre-irradiation, decrease stepwise with different decay constants. At first, they decay rapidly, but afterwards they decay more and more slowly. (2) At different temperatures, free radicals have different decay constants but they have almost the same decay characteristics as described above. (3) Sampling pre-irradiated PE films during the fast decay period may cause a pronounced error in the percentage graft, so that the standard deviation is much larger. On the other hand, sampling pre-irradiated PE films during the slow decay period greatly improves the precision of the percentage graft, so the standard deviation is much smaller. (4) For the above reason, the optimum storage time of pre-irradiated PE films at 5°C may be considered to be in the range of 48–120 h after the free radicals have been created. The results of graft copolymerization of pre-irradiated PE films with 4-vinylpyridine in the gaseous phase at different temperatures showed that: (1) At higher grafting temperatures, the grafting rate of 4-vinylpyridine onto PE films is much higher than the swelling rate of PE films by monomer vapor. As the grafted macromolecular chains propagated surround the activated sites very quickly, there is a local thickening effect, and the resulting graft copolymer looks rough. (2) At lower grafting temperatures, the grafting rate of 4-vinylpyridine seems to be comparable with the swelling rate of PE films by monomer vapor; an even and smooth copolymeric film can be obtained.