The present invention relates to a proton-conducting polymer membrane comprising polyazoles which is obtainable by a process comprising the steps A) preparation of a mixture comprising polyphosphoric acid, at least one polyazole and/or one or more compounds suitable for forming polyazoles under the action of heat in step B), B) heating of the mixture obtainable according to step A) to temperatures up to 400° C. under inert gas, C) application of a layer to a support using the mixture from step A) and/or B), D) treatment of the membrane formed in step C) until it is self-supporting, wherein the treatment of the membrane is carried out using a hydrolysis liquid comprising oxo acids of phosphorus and/or oxo acids of sulfur.