NOVELTY - A method for optimizing the ionic conductivity (IC) of an ionic conductor membrane (ICM), formed from material (I) allowing introduction of water vapor, involves (a) introducing a pressurized gas stream containing water vapor into the membrane and (b) optimizing the IC at a particular temperature as a function of the water vapor partial pressure, this partial pressure being at least 1 bar.    USE - Especially for optimizing the ionic conductivity of a ionic conductor membrane (ICM) in an electrolytic hydrogen production device, a hydrogen purification or pumping device or a fuel cell (all claimed). More generally the method is useful for improving the performance of ceramic ICM's. The process is particularly applied to ICM's for producing hydrogen on an industrial scale by high temperature electrolysis of water.    ADVANTAGE - The conductivities of the ICM's are optimized so that ohmic losses within the membrane are restricted.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) an electrolytic hydrogen production device for carrying out the IC optimization process, comprising an electrolyte in the form of a recrystallized ICM of (I), an anode, a cathode, a generator for applying a potential difference between the anode and cathode and a system for introducing water vapor under pressure into (I) via the anode or cathode;    (2) a hydrogen purification device for carrying out the process, comprising a proton filtration membrane formed from (I) and a system for introducing water vapor under pressure into (I) such that the potential at the permeate side is greater than that at the input side;    (3) a hydrogen purification or pumping device for carrying out the process, comprising a proton filtration membrane formed from (I), an anode, a cathode, a generator for polarizing the anode positively and the cathode negatively and a system for introducing water vapor under pressure into (I) via the cathode; and    (4) a hydrogen fuel cell for carrying out the process, comprising an electrolyte in the form of a recrystallized ICM of (I), an anode, a cathode and a system for introducing water vapor under pressure into (I) via the anode or cathode, provided that if introduction is via the anode then the water vapor pressure is such that the potential of the cathode is higher than that of the anode.