Abstract: An improved method for manufacturing membrane battery separators by grafting a vinyl monomer onto a polyolefin film. The method includes irradiation of the film by ultraviolet rays and subsequent grafting of the said film in a five-layer sandwich. The sandwich comprises the irradiated polyolefin film which is covered on each side with a layer of a cellulosic tissue soaked with the monomer or a solution of the monomer, and each cellulosic layer on its turn is covered by an inert polymeric film. The layers of the sandwich are not limited in length and the sandwich is transported at a constant speed over the surface of a heater, maintaining the sandwich at a constant temperature in the range 85.degree. to 95.degree. C. with an accuracy of .+-.1.degree. C. The grafting of the polyolefin film in the sandwich proceeds continuously and isothermally, while it is in contact with the heater surface under a pressure of 10 to 30 g/cm.sup.2 during 10 to 45 minutes.