TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Materials: The polymer compound also contains a plasticizer such as a phosphate compound e.g. triphenyl phosphate and/or tricresyl phosphate. The Ar1 is a divalent aromatic unit of formulae (II)-(XI).FORMULAEThe proton-conductive substituent group is a sulfonic acid group. The amount which can oxidize to sulfoxide or sulfone among sulfide units of the polymer compound is 0-10 mol% based on the total weight of repeat units. At least one radiation selected from gamma-rays, electron beam and ion beam is irradiated at the film. The irradiation amount is 10-1000 kGy. The ion exchange capacity is 0.5-8 milli-equivalents per gram. The proton conductivity at room temperature is at least 0.01 S/cm. The thickness of the proton conductive polymer film is 5-200 micronsm. The sulfonating agent is chlorosulfonic acid, SO3-triethyl phosphate, concentrated sulfuric acid, fuming sulfuric acid and/or trimethylsilyl-chlorosulfate. The amount of the sulfonating agent is 0.5-10 milliequivalents based on aromatic units of the polymer compound. The sulfonating agent is used in the presence of a solvent and the concentration of the sulfonating agent in the mixed solution of sulfonating agent and solvent is 0.1-10 weight%. The solvent is 1,1,2,2-tetrachloroethane, 1,1,1,2-tetrachloroethane, 1,1,1-trichloroethane, 1,2-dichloroethane, trichloroethylene, tetrachloroethylene, dichloromethane and/or chloroform. EXAMPLE - A polyphenylene sulfide polymer film was used which uses repeat unit (I) where Ar1 was a group of formula (II) and x was 0. A chlorosulfonic acid solution was formed by adding 50 g dichloromethane and 0.117 g chlorosulfonic acid into a 50 ml screw tube. The polymer film (0.117 g) was immersion contacted to the chlorosulfonic acid solution and left for 20 hours at room temperature. The proton conductive polymer film was then recovered. It was washed so it became neutral with ion exchanged water, and immersed in 50 ml ion exchanged water and left for 20 hours at room temperature. It was reduced pressure dried at 80degreesC for 3 hours to give a proton-conductive polymer film which was a sulfonated polyphenylene sulfide. The ion exchange capacity of the film was 1.47 milliequivalents per gram and the proton conductivity at room temperature was 5.2 x 10-2 S/cm. The film-forming handling properties were good and it was insoluble in methanol and N-methyl pyrrolidone.