EMF of the hydrogen isotope cell has been applied to the sensing of hydrogen isotopes. In this paper, the atomic hydrogen isotope composition and the total pressure of the hydrogen isotopes were determined simultaneously by use of two electrochemical cells with different kind of proton-conducting electrolytes. Among the several proton conductors, CaZr0.9In0.1O3-x and SrCe0.95Yb0.05O3-z were chosen for the sensor. The difference in EMFs of the two proton conductor cells was independent of the total pressure of hydrogen isotopes and, thus, served to determine the atomic hydrogen isotope compositions in gases. (C) 2004 Elsevier B.V. All rights reserved.