An electrochemical system comprising: - a device forming a high-temperature steam electrolyzer or a high-temperature fuel cell, the device comprising a stack of elementary electrochemical cells, each comprising two electrodes (12, 14), and an electrolyte interposed between the two electrodes (12, 14), said cells being electrically connected in series and the stack comprising two electric terminals (30, 32) of generation of an electric current I in the stack crossing said cells; - a gas supply capable of supplying a gas flow rate D G: ▪ the electrodes of the stack forming steam cathodes if the device is a high-temperature electrolyzer; or ▪ the electrodes of the stack forming hydrogen anodes if the device is a high-temperature fuel cell; - an electric circuit connected to the electric terminals (30, 32) of the stack to control the electric current I in the stack; characterized in that the electrochemical system further comprises: - a module for measuring an electric voltage V measured across the stack; - a first regulator for controlling the electric current I in the stack to regulate the measured voltage on a predetermined voltage set point V 0; - a module for measuring a rate τ measured, said rate τ measured being: ▪ a rate of conversion of steam into hydrogen if the device is a high-temperature electrolyzer; or ▪ a rate of hydrogen utilization if the device is a high-temperature fuel cell; - a second regulator for controlling the gas flow rate D G of the gas supply to regulate the measured rate τ measured on a predetermined rate set point τ 0. - a device forming a high-temperature steam electrolyzer or a high-temperature fuel cell, the device comprising a stack of elementary electrochemical cells, each comprising two electrodes (12, 14), and an electrolyte interposed between the two electrodes (12, 14), said cells being electrically connected in series and the stack comprising two electric terminals (30, 32) of generation of an electric current I in the stack crossing said cells; - a gas supply capable of supplying a gas flow rate D G: ▪ the electrodes of the stack forming steam cathodes if the device is a high-temperature electrolyzer; or ▪ the electrodes of the stack forming hydrogen anodes if the device is a high-temperature fuel cell; ▪ the electrodes of the stack forming steam cathodes if the device is a high-temperature electrolyzer; or ▪ the electrodes of the stack forming hydrogen anodes if the device is a high-temperature fuel cell; - an electric circuit connected to the electric terminals (30, 32) of the stack to control the electric current I in the stack; - a module for measuring an electric voltage V measured across the stack; - a first regulator for controlling the electric current I in the stack to regulate the measured voltage on a predetermined voltage set point V 0; - a module for measuring a rate τ measured, said rate τ measured being: ▪ a rate of conversion of steam into hydrogen if the device is a high-temperature electrolyzer; or ▪ a rate of hydrogen utilization if the device is a high-temperature fuel cell; ▪ a rate of conversion of steam into hydrogen if the device is a high-temperature electrolyzer; or ▪ a rate of hydrogen utilization if the device is a high-temperature fuel cell; - a second regulator for controlling the gas flow rate D G of the gas supply to regulate the measured rate τ measured on a predetermined rate set point τ 0.