The present invention relates to an acid-doped polymer membrane based on polyazoles. The acid-doped polymer membrane can be used in a variety of applications because of its excellent mechanical properties and is useful as polymer electrolyte membrane (PEM) in PEM fuel cells. A doped polymer membrane based on polyazoles is obtained by a process comprising the steps of: A) casting a film using a solution of polymers based on polyazoles in a polar, aprotic organic solvent; B) drying the film formed in step A) until it is self-supporting; C) treating the film obtained in step B) with a treatment liquid at a temperature in the range from room temperature to the boiling point of the treatment liquid; D) drying and/or dabbing the film treated according to step C) to remove the treatment liquid from step C); and E) doping the film treated according to step D) with a doping agent.