The electromotive force (EMF) of hydrogen isotope cell with a proton‐conducting solid electrolyte was investigated at 500 ∼ 900°C. This kind of cell gave steady and reproducible EMF for which the side electrode was positive. The EMF could be explained by the two factors, i.e., the difference in mobilities of proton and deuteron in the electrolyte and the electrode reactions of and gases. The EMF of the cell for mixed isotope gases was nearly linear with the composition of hydrogen isotopes with a slight deviation and was consistent with such a mechanism of EMF of hydrogen isotope cell. © 1999 The Electrochemical Society. All rights reserved.