It is very important to understand temperature distribution in electrolytic cells quantitatively for designing their structure or establishing their electrolytic conditions. For example, if Joule heat or the other kinds of heat evolved from electrode reaction is not estimated properly, the temperature of various materials such as electrodes, electrolyte, or membranes, rises and in some case it leads to the destruction of the cell[l]. Conversely, if insufficient heat is supplied to the electrode region, where the reaction is endothermic, the temperature may fall, thus increasing the voltage required for the electrolysis. Besides, in molten salt systems, even freezing of the electrolyte in a cell might occur under extreme conditions. For such reasons, the quantitative evaluation of single electrode heat is very important from the engineering viewpoint. Especially it is indispensable for the optimization of electrolytic conditions. But so far little attention has been paid to the single electrode heat in spite of its importance and most attention has been paid to the total enthalpy change in the electrolysis reactions, instead. This paper reports the single electrode heats of H, gas evolution in NaOH solutions at various concentrations up to 10N. Especially the data on NaOH aqueous systems at high concentration is important from the-engineering viewpoint because such solutions are to be used in water electrolyzers and brine electrolyzers, etc. Since our last work[Z] showed that the more reliable data can be obtained by thermoelectric power measurement than by direct calorimetric measurement, in this work the former method was adopted to determine the single electrode heat of hydrogen gas electrode. Some general concepts on the relation between thermoelectric power and single electrode heat will also be presented in this paper in addition to the explanations and discussions on thermoelectric power measurement and its results.