The equilibrium potential of the H-2/H- couple has been measured in a molten LiCl-KCl-LiH system at 643-883 K. The Nernst relation holds for the potential at hydrogen pressure, p(H 2), of 0.1-1.0 atm and H- ion concentration, XH-, of 0.002-0.080 (anion fraction). The standard formal potential of H-2/H-, E-H2/H-(0'), is evaluated to be 0.258+/-0.002 V (vs. Li+/Li) (p(H 2) = 1 atm, XH- =1) at 673 K.(0')(EH2/H-) changes linearly with temperature at 643-883 K, having the gradient of -0.575+/-0.011 mV K-1. Thermodynamic quantities for formation of LiH in the melt are also estimated. When the activity coefficient of H- ion, gamma(H-), is defined as unity in pure liquid LiH (XH- = 1), gamma(H-) in the dilute H- region (0.002 less than or equal to XH- less than or equal to 0.080) is evaluated to be 7.1. (C) 2002 The Electrochemical Society.