TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Film: The aromatic vinyl group compound is selected from styrene, alpha-methyl styrene, paramethyl styrene, vinylnaphthalene and indene derivatives. The ion exchange capacity of sulfonated body of isobutylene group copolymer is 0.50 meq/g or more. EXAMPLE - Isobutylene group block copolymer (styrene-isobutylene-styrene triblock copolymer) of number average molecular weight 104000, styrene content of 30 weight% (wt.%), was used. 10 g of the copolymer was added to a 500 ml separable flask, 50 g each of dichloroethane and cyclohexane were added and stirred and copolymer was dissolved completely. 23.7 ml of dichloromethane was added to a 100 ml flask separately to which 4.95 g acetic anhydride was added and the flask was frozen. 1.68 g of sulfuric acid was added and stirred at 10degreesC or less. A solution of sulfonated agent of 30 ml was obtained. The solution was dropped to the solution of isobutylene group block copolymer at room temperature, stirred for 2 hours at 60degreesC under nitrogen air current. A slightly yellow colored solution was obtained, to which 5 ml methanol was added and the solvent was distilled off. The obtained reaction mixture was added to 1 L water and the formed rubber like substance was isolated. Cyclohexane was dispersed and kept for 1 night and filtered. After carrying out suction filtration, the product was dried in vacuum of 50degreesC and the sulfonated body of isobutylene group block copolymer was obtained. 15 wt.% toluene solution of sulfonated isobutylene group block copolymer was prepared and casted on a glass plate to a thickness of 500 mum and vacuum dried for 14 hours at 120degreesC. A film of thickness 70 mum was obtained. The ion exchange capacity of the film before and after Fenton reaction were 0.54 meq/g and 0.31 meq/g, respectively. The ion exchange capacity percentage reduction was 45%. The film had proton conductivity sufficient for the film for fuel cells. Excellent chemical stability was observed.