A process for preparing mechanically stabilized polyazoles, comprising the following steps: I) treating at least one polyazole having at least one amino group in a repeat unit with a solution comprising (i) at least one strong acid and (ii) at least one stabilizing reagent, the total content of stabilizing reagents in the solution being in the range from 0.01 to 30% by weight, II) performing the stabilization reaction directly and/or in a subsequent processing step by heating to a temperature greater than 25° C., the stabilizing reagent used being at least one polyvinyl alcohol. The polyazoles thus obtainable are notable especially for a high conductivity and a very good mechanical stability. They are therefore especially suitable for applications in fuel cells.