NOVELTY - The process of electrolyzing water at high temperatures implemented by a cell stack reactor, comprises simultaneously circulating water vapor (12, 13) to each cathode and to each anode as a leaching gas for evacuating oxygen product, supplying electrical power having a constant electrical voltage U0 across the terminals of the stack upon starting the electrolysis. The reactor comprises a stack of N basic electrochemical cells formed of a cathode (11), an anode (9) and an electrolyte intercalated between the cathode and anode, and a plate (8) interconnected between adjacent cells.    USE - The process is useful for electrolyzing water at high temperatures implemented by a cell stack reactor, which is useful in an assembly for production of hydrogen (claimed).    ADVANTAGE - The process ensures high production of hydrogen with improved efficiency, avoids the breakage of cells and the complete destruction of the stack of cells during the electrolysis process, and maintains the efficiency of cell stack reactor during aging.    DETAILED DESCRIPTION - The process of electrolyzing water at high temperatures implemented by a cell stack reactor, comprises simultaneously circulating water vapor (12, 13) to each cathode and to each anode as a leaching gas for evacuating oxygen product, supplying electrical power having a constant electrical voltage U0 across the terminals of the stack upon starting the electrolysis. The reactor comprises a stack of N basic electrochemical cells formed of a cathode (11), an anode (9) and an electrolyte intercalated between the cathode and anode, and a plate (8) interconnected between adjacent cells and in electrical contact with the electrodes. The temperatures of the water vapor at the anode and cathode is lower than the high temperatures at which the electrolysis is carried out, and the water vapor circulating at the anode is at an overpressure of 30 mbar with respect to the cathode. The voltage level across each single unit cell is 1.3 V. The rate of conversion of initial hydrogen is 100%. The flow of water vapor is increased at each cathode, when the rate of conversion of initial hydrogen is predetermined at the outlet of each reduced cathode. The electrolysis process is carried out at high temperatures of 700-1000 degrees C. An INDEPENDENT CLAIM is included for a device for electrolysis of water at high temperatures.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic sectional view of a cell stack reactor.    Interconnection plate (8)    Anode (9)    Cathode (11)    Water vapor (12, 13)    Breakage part. (17)