AbstractThe radiation initiated grafting of N‐vinylpyrrolidone (NVP) onto copolymers of vinyl fluoride/vinylidene fluoride (VF/VF2) with compositions of 75/25 mol‐%, 50/50 mol‐% and 20/80 mol‐% VF/VF2 has been investigated. The active species, initiating the grafting, have been created by accelerated electrons (160 keV) applying preirradiation technique in vacuo. The time conversion curves present a characteristic temperature dependence. Increasing grafting temperature rises the initial grafting rate. The final (saturation) grafting yield increases with temperature up to 333 K, and then declines. This behaviour was described by the influences of glass transition on the rate of termination in the semicrystalline trunk polymers. The grafting rate (at constant irradiation dose and grafting temperature) decreases with increasing VF2 content in the copolymers. The reason is discussed by means of a semiquantitative mathematical analysis of the graft kinetics applying “non‐steady‐state”‐conditions. The grafting rate does not depend on dose rate, but strongly depends on irradiation dose passing a maximum. This maximum has been interpreted by means of “Differential‐Interference‐Contrast”‐microscopy. The enthalpies of melting ΔHm of the semicrystalline copolymers decrease after grafting with increasing grafting yield. This change is due to the partial destroying of crystalline regions in the trunk copolymers caused by stresses arising from grafted polymer chains. The membrane resistance of films grafted through is <10 Ω cm2. The membranes proved to be permeable for urea.