Electrochemical behavior of hydride ion was studied in three kinds of eutectic melt of alkali chlorides; LiCl-KCl, LiCl-CsCl, and LiCl-KCl-CsCl. The equilibrium potential of H 2 /H - electrode was obtained in these three molten alkali chlorides. The effects of the concentration of H - and the pressure of H 2 on the potential were explained by the Nernst equation. Temperature dependence of the standard electrode potential was approximately expressed by a straight line for these melts over a temperature range from 600 to 773 K. The diffusion coefficients of hydride ion were obtained in these three melts by voltammetric analysis using copper electrode, because hydrogen solubility in copper is very low and the accurate oxidation peak of hydride ion was obtained.