Sulfonated cation exchange membranes have been prepared by the postradiation graft polymerization of styrene on a 20-mu m-thick low-density polyethylene film and the subsequent sulfonation of the grafted polystyrene. The distribution of sulfonated polystyrene in the membranes at varying degree of grafting has been described using IR spectroscopy data. It has been shown that the permeability of the membranes to ethane and ethylene decreases with the increasing degree of grafting. The use of gas mixtures with a low ethylene content leads to the suppression of the ethane permeability coefficient and an increase in the separation factor. The highest separation factor of 57 has been achieved at the 140% degree of grafting of the polymer from a mixture containing 5% ethylene.