TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The electrolyte membrane is prepared by subjecting base matrix comprising fluorine-containing polymer, an olefinic polymer or an aromatic polymer to radiation-induced graft polymerization of monomer having monomeric acrylic acid derivative or monomeric vinylketone derivative as a framework; and introducing sulfonic acid groups into the graft chains on the resulting polymer substrate. The sulfonic acid groups are introduced using a sulfonating agent complexed with coordinating compound (preferably dioxane, dimethyl ether, diethyl ether, dimethylformamide or pyridine). The introduction of sulfonic acid groups is followed by alkali or acid treatment to eliminate a part of the carboxylic acid or ketone in the graft chains to achieve higher resistance to hot water and oxidation.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The polymer substrate and/or the sulfonic acid group containing polymer chains that have been grafted on the substrate have a crosslinked structure. EXAMPLE - An ethylene-tetrafluoroethylene (ETFE) film with thickness of 50 mu m was placed in glass ampule equipped with cock, which was degassed and purged with argon gas at one atmosphere. Under this condition, the ETFE film was irradiated with 20 kGy of gamma -rays (dose rate: 20 kGy/hours) at room temperature. After the irradiation, the vessel was evacuated and charged with 20 ml of preliminarily argon-purged 30 vol.% solution of monomeric methyl acrylate to soak the ETFE film. The interior of the vessel was displaced with argon and then closed with the cock, followed by reaction for 3 hours in constant temperature (60 degrees C) water bath. After the reaction, the ETFE film was washed with toluene to remove the unreacted monomer and any resulting homopolymer; subsequent drying afforded an ETFE film with 91% grafting. This film as obtained by graft polymerization was put into a liquid mixture of 0.2M chlorosulfonic acid diluted with 1,2-dichloroethane and 0.2M dioxane and subjected to reaction at 60 degrees C for 6 hours, followed by thorough washing with water to prepare an electrolyte membrane. The electrolyte membrane was soaked in 0.2M aqueous sodium hydroxide at 60 degrees C for 24 hours to effect alkali treatment. Since the alkali treatment changed the ion-exchange groups in the membrane to sodium form, the membrane was then treated with 1M aqueous hydrochloric acid to become proton form before it was finally washed with pure water for more than a day. This membrane had weight change from sulfonation of 7.15%, ion-exchange capacity of 1.35 meq.g, oxidation resistance of 99%, electrical conductivity of 0.11 S/cm, and hot water resistance of 100%.