The oxygen ion conductor yttria-stabilized zirconia (YSZ), which is usually used as the electrolyte of SOFC, operates at high temperatures of about 1000 degreesC. The recent trend in developing SOFC is to reduce the operating temperature. Proton conducting cerates may allow the intermediate temperature operation for SOFC applications. Rare-earth-doped BaCeO3 electrolytes with the perovskite structure present good protonic conductivities at moderate temperatures but rather poor chemical stability and endurance for moisture. Barium zirconate, in contrast, is a rather stable material but exhibits low protonic conductivity. We then focused on a practical protonic conductor of BaZr0.4Ce0.4In0.2O3 (BZCI) that has a relatively high durability against moisture and good protonic conductivity. However, little is known about its stability and electrochemical properties in reducing hydrogen. In this work, the electrochemical properties of BZCI as SOFC electrolytes were investigated in concentration cell and fuel cell operations. From the results of concentration cell measurements, it was revealed that BZCI has good proton conductivities in hydrogen-rich atmospheres and behaves as a protonic and oxide ionic conductor in oxygen-rich atmospheres, with some extent of electronic conductivity, which lowers its ionic transport number. Open circuit voltage (OCV) measurements in Fuel cell operations showed that OCV value of a Pt\ BZCI\ Pt cell is about 870 mV at 800 degreesC and 1020 mV at 600degreesC. (C) 2004 Elsevier B.V. All rights reserved.