TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The solution in step (i) comprises at least one strong protic acid, preferably phosphoric acid and/or sulfuric acid. The phosphoric acid comprises polyphosphoric acid, phosphonic acid, orthophosphoric acid, pyrophosphoric acid, triphosphoric acid, metaphosphoric acid or its derivatives, preferably organic derivatives e.g. cyclic organophosphoric acids or their derivatives e.g. acid esters.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The polyazole exhibits a molecular weight, which is measured as intrinsic viscosity of at least 1.8 dl/g. The polyvinyl alcohol comprises (in wt.%, based on its total weight) (A) ethyl structural units of formula (-CH(OH)-CH(R1)-) (I) (1-99.9), (B) formic acid ethyl ester structural units of formula (II) (0-99) and (C) ethyl structural units of formula (-C(R5)(R6)-C(R3)(R4)-) (III) (0-70). The polyvinyl alcohol comprises (I) and/or (II) in a total amount of greater than 50 wt.%, based on its total weight, preferably (I). The polyvinyl alcohol exhibits a molar mass of 10000-200000 g/mole. The membrane comprising polyazole polymer (less than 0.5 wt.%) exhibits a solubility in 99% phosphoric acid at a temperature of 85-120 degrees C.R = H or CH3;R2 = H or 1-6C alkyl; andR3-R6 = groups having molecular weight of 1-500 g/mol.Preferred Method: The method comprises: either (a1) producing a film comprising at least one polyazole having at least one amino group in a repeating unit, (b1) treating the film obtained in step (a1) with the solution comprising (i) and (ii), (c1) carrying out the stabilization reaction of membrane obtained in step (b1) directly or in a subsequent step by heating the membrane to a temperature of greater than 25 degrees C, and (d) optionally and additionally doping the membrane obtained in step (b) with a strong acid or concentrating the already present strong acid by removing water; or (a2) preparing a solution or a dispersion of at least one polyazole with at least one amino group in a repeating unit, in orthophosphoric acid and/or polyphosphoric acid, (b2) mixing the solution or the dispersion obtained in step (a2) with the solution comprising (i) and (ii), (c1) carrying out the stabilization reaction of the solution or the dispersion obtained in step (b1) either directly or in a subsequent step by heating the solution or the dispersion to a temperature of greater than 25 degrees C, and (d1) optionally and additionally doping polymer obtained in step (c1) with a strong acid or concentrating the already present strong acid by removing water. EXAMPLE - No suitable example given.