A proton-conducting electrolyte membrane is disclosed, comprising at least one base material and at least one dopant, which is the reaction product of an at least dibasic inorganic acid with an organic compound, comprising one acidic hydroxyl group, or the condensation product of said compound with a polybasic acid. The membrane may be produced by a single step method, which avoids the use of dangerous materials and environmental pollutants. Subsequent doping of the membrane, e.g., in conjunction with assembly of the membrane electrode assembly (MEA) is not excluded. The electrolyte membrane has a high and constant mechanical stability and flexibility, excellent chemical and thermal stability and a high and constant conductivity. The membrane may be used in a fuel cell in a wide temperature range from 50.degree. C. to more than 200.degree. C., for example, whereby the fuel cell has a high and constant power level over the entire temperature range.