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Proton Exchange Membranes for fuel cell applications were synthesized by radiation grafting of styrene on FEP (Fluoro-Ethylene-Propylene) and subsequent sulfonation of the films. Grafting in film form overcomes the problem of shaping the polymer in a thin foil and makes this process a potentially cheap and easy technique for the preparation of solid polymer fuel cell electrolytes. The properties of the synthesized membranes can be tailored by varying the degree of grafting and crosslinking. Specific resistance of membranes as low as 3.2 Ωcm at 20 °C and thicknesses of less than 100 μm (swollen) could be achieved. The performance of these membranes was tested in H2/O2 fuel cells. With selected membranes a polarization characteristic close to Nafion 117 was measured. Long time testing proved steady performance for high grafted membranes over periods of more than 300 hours at a cell temperature of 60 °C.