NOVELTY - Proton-conducting, flexible electrolyte membrane for a fuel cell, which is impermeable for the reaction components of the fuel cell reaction, comprises a permeable composite of a flexible, porous ceramic substrate and a porous ceramic material, which is infiltrated with proton-conducting material suitable for selective conduction of protons through the membrane.    USE - The electrolyte membrane is used in a fuel cell, especially a direct methanol fuel cell or reformate fuel cell, and for producing a membrane electrode unit, fuel cell or fuel cell stack; and the membrane electrode unit is used in a fuel cell of these types (all claimed). The mobile or stationary system preferably is a vehicle or domestic energy system (claimed).    ADVANTAGE - Existing electrolyte membranes either cannot be used at temperatures above 100degreesC, as they are too permeable for methanol and allow crossover to the cathode side, or are subject to short circuits under practical conditions. The present flexible membranes have high proton conductivity and much lower water vapor permeability than polymer membranes and can be made thinner than the latter. They give membrane electrode units with low total resistance. Their mechanical properties, e.g. tensile strength and flexibility, make them suitable for use under extreme conditions, as encountered in vehicles. They tolerate operating temperatures of over 80degreesC, avoid short circuit and cross-over problems and can be produced easily.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) production of the electrolyte membrane;    (2) flexible membrane electrode unit for a fuel cell with electrically conducting anode and cathode layers on opposite sides of the membrane;    (3) production of this unit;    (4) compositions comprising a condensable component, which, by condensation, imparts proton conductivity to an anode or cathode layer of such a unit, a catalyst (precursor) for the anode or cathode reaction in a fuel cell and optionally a catalyst support, pore former and/or additives to improve the foaming property, viscosity and adhesion;    (5) fuel cells with the electrolyte membrane;    (6) fuel cells with the membrane electrode unit; and    (7) mobile or stationary systems with membrane electrode unit, fuel cell or fuel cell stack containing electrolyte membrane or membrane electrode unit.