The EMFs of hydrogen isotope cells with H-2-D-2 gas mixtures against H-2 and D-2 reference gases were measured at 700 degreesC using CaZr0.90In0.10O3-alpha as a proton-conducting solid electrolyte. Steady electromotive forces (EMFs) with quick response were obtained. The EMFs were almost linear functions of the gas composition of the H-2-D-2 mixtures and were consistent with those theoretically estimated. Such defined EMFs can he used to monitor H-2/D-2 ratio in hydrogen isotope mixtures.