TECHNOLOGY FOCUS - POLYMERS - Preferred Components: Component (A) comprises repeat units of formula -(Ar1-O-Ar2-CO)- (I), in whichAr1, Ar2 = divalent aromatic or heteroaromatic groups, substituted with groups of formula -(SO3)wM and optionally substituted with one or more inert organic groups;M = a w-valent metal cation, ammonium or especially H;w = a whole number, especially 1 or 2. (A) has an ion exchange capacity of 1.6-2.9 meq (-SO3H)/g polymer. Component (B) comprises units of formula (II);Ar = a tetravalent aromatic residue;Ar' = arylene;R = H or an inert monovalent organic residue.The content of (B) depends on the degree of sulfonation of (A) and is calculated as wt% (B) = 9.4x - 12.4 +/- (9.4x - 12.4) x 0.5, where x = the ion exchange capacity of (A).Preferred Composition: The types and amounts of (A) and (B) are selected to enable the preparation of a solution in NMP with a viscosity of 500-5000 mPa.s (measured at 80degreesC with a Couette-type rotational viscometer).Preferred Product: Claimed membranes show a thickness of at least 5 (preferably at least 30) microns, an ionic conductivity (measured in contact with liquid water at room temperature, using 4-pole impedance spectroscopy with a phase angle of less than 1degrees) of not less than 50 mS/cm, high stability towards boiling water (72 hours at 100degreesC), an elastic modulus (increase in tangent at 1.2 MPa) of at least 600 MPa in the dry state (23degreesC/50% RH) or at least 90 MPa in water at 60degreesC (with an elongation at break of more than 200 % in water at 60 degreesC), a residual solvent content of less than 0.5 wt% and a proton conductivity of more than 50 mS/cm in contact with water at room temperature.Preferred Process: The membrane is purified to remove solvent and any other water-soluble impurities by washing in water or dilute (0.1-20%) acid, e.g. nitric, sulfuric or phosphoric acid, and then dried by heating until the solvent content is less than 0.5 wt%. The membrane may be produced by applying the solution of (A) (salt) and (B) to an absorbent non-woven fabric and then removing the solvent by evaporation. Preferred Definitions:    Ar1, Ar2 = naphthylene or preferably phenylene, optionally substituted with 1-4 amino, alcohol, alkyl, aryl, sulfonyl, phosphonyl, carbonyl, nitro or carboxylic acid groups, and/or the nitrogen atoms of the polybenzimidazole may be substituted with these groups.    Ar = benzene-1,2,4,5-tetrayl or biphenyl-3,4,3',4'-tetrayl;    Ar' = 1,3- or 1,4-phenylene;    R = H.    EXAMPLE - Sulfonated PEK (S-PEK) was obtained by mixing 3.49 kg 98% sulfuric acid with 400 g Victrex PEK (RTM: polyether-ketone), heating at 50degreesC to give a clear red solution, adding 2.4 kg oleum (20% free sulfur trioxide), sulfonating to an ion exchange capacity of 2.12 meq (SO3H)/g polymer, precipitating in water, washing neutral and drying at 60-120degreesC. Polymer blends were obtained by converting the S-PEK into its sodium salt (1-M sodium hydroxide at 40-80degreesC), dissolving the dry salt in N-methylpyrrolidone (NMP), mixing the 15-20% solution with a 15% solution of polybenzimidazole (PBI) in dimethylacetamide (obtained as described in EP 816415), stirring for 1 hour and filtering at 80degreesC. The solution obtained was spread on a glass plate and dried overnight at 80-140degreesC to give a membrane which was treated with 1-M sulfuric acid at 40degreesC (to remove sodium), washed with water and dried. A membrane with a PBI content of 5 wt% showed proton conductivities (4-pole measurements, in contact with water) of 0.056282855 S/cm (initial, 23degreesC), 0.212844755 S/cm (80degreesC) and 0.116734811 S/cm (final, 25degreesC), and elastic moduli of 1140 (695) MPa at 23degreesC and 50% RH or 536 (7) MPa at 60degreesC in water, with corresponding elongations at break of 72 (121) % and 284 (200) %. Values in brackets are for an S-PEK membrane with no PBI.