TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Process: By stretching under heating, the thickness of the membrane is made to 3-70 microns, and the fluorocarbon polymer is fibrillated, so that number of fibrils having a fibril-fiber diameter of at most 1 microns accounts for at least 70% of the total number of fibrils. The mixture of PFC polymer and the FC polymer, is formed into a membrane and the precursors of sulfonic acid groups in the PFC are hydrolyzed and converted to acid forms, followed by stretching.Alternatively, the membrane is a laminate comprising at least two cation exchange layers made of a PFC polymer of which at least one layer is reinforced with fibril-form polymer reinforcement and at least one layer is not reinforced.Preferred Materials: The support film for stretching is made of a polyethylene terephthalate (PET) or a polypropylene and the stretching (biaxial stretching) is carried out at 40-200degreesC. The amount of reinforcement is 0.5-15 mass% of total mass of electrolyte membrane. The reinforcement contains at least 80% based on total mass, of polytetrafluoroethylene (PTFE) or a copolymer containing 95 mol% (based on the copolymer) of polymer units derived from tetrafluoroethylene. Preferably, the PFC polymer is a copolymer having polymer units derived from formula (I) and polymer units derived from formula (II):X = fluorine atom or trifluoromethyl group;m = integer from 0-3;n = integer from 0-12;p = 0 or 1.Proviso if n is 0, p is 0. EXAMPLE - 9730 g of a powder of copolymer A (ion exchange capacity 1.1 meq/g dry resin), from tetrafluoroethylene polymer units and polymer units of formula (III) (fibril-forming polymer) and 270 g of PTFE powder Fluon CD-1, were mixed to obtain 9500 g of pellets.    A membrane of thickness 250 microns was obtained by extrusion and sandwiched between 2 amorphous PET films (thickness 200 microns) as support films for stretching. A heat-roll press at 80degreesC was performed to obtain a laminated membrane. It was then biaxially-stretched at 85degreesC with stretch ratio of 1.4 in axial directions to obtain a stretched membrane having thickness of 120 microns, sandwiched between two support films.    The laminated membrane was heat-treated at 180degreesC, support films were removed and the membrane was hydrolyzed using an aqueous solution containing dimethyl sulfoxide and sodium hydroxide, to convert to acid-forms with hydrochloric acid. A membrane of thickness 120 microns was obtained. The process was repeated to obtain a fibril-reinforced membrane of thickness 30 microns.    The thickness of the film was uniform and variations on the surface were within +/-3 microns. The number of fibrils having fiber diameter of at most 1 microns in an area of 5 microns2 was 98% of the total number of fibrils. Tear strength of the membrane was 2.5 N/mm in machine direction and 15 N/mm in transverse direction. Resistivity was 5 OMEGA/cm.    When the membrane was assembled in a fuel cell, the terminal voltage at 80degreesC and current density of 1 A/cm2, was 0.62 V. Continuous operation of the fuel cell was carried out at 80degreesC at the same current density and no change in terminal voltage was observed even after 1000 hours.