Proton-conducting polymer electrolyte membrane based on a polyazole salt of an inorganic or organic acid which is doped with an acid as electrolyte, wherein the polyazole salt of the organic or inorganic acid has a lower solubility in the acid used as electrolyte than the polyazole salt of the acid used as electrolyte, a process for producing the inventive proton-conducting polymer electrolyte membrane, a membrane-electrode assembly comprising at least two electrochemically active electrodes which are separated by a polymer electrolyte membrane, wherein the polymer electrolyte membrane is a proton-conducting polymer electrolyte membrane according to the invention, and a fuel cell comprising at least one membrane-electrode assembly according to the invention.