The following hydrogen and/or oxygen concentration cell was constructed, and the electromotive force (emf) was measured over a temperature range from 1203 to 1523 K under a hydrogen-rich atmosphere and over 773 to 1073 K under an oxygen-rich atmosphere. (ref., -); M-1 (p'H-2; p'O-2 )|CaZr0.9In0.1O3-delta|M-2(P ''(H2), P ''(O2))(+, work). The relationship between the measured emfs and the activities of the hydrogen and/or oxygen were represented by the following Schmalzried-type equation. E = -RT/F InP ''(1/2)(H2) + w(1)/P'(1/2)(H2) + w(1) - RT/F InP ''(1/4)(O2) + w(2)/P'(1/4)(O2) + w(2), where w(1) was a parameter which depended on the temperature and p ''(O2), and w(2) was one which depended on the temperature and p'(H2). In this study, it was ascertained that the obtained equation could be put to practical use. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3571971] All rights reserved.