Y-doped BaCeO3 (BCY), a solid state ion conductor which exhibits mixed proton and oxide ion conductivity under some conditions, was used as an electrolyte in a solid oxide fuel cell (SOFC). The conductivities of 10% and 25% Y-doped BaCeO3 (BCY10 and BCY25) were measured under a single gas atmosphere of 5% hydrogen in argon (5% H2/Ar), air and pure argon (Ar). Furthermore, a dual-chamber SOFC reactor was used to investigate the conductivities of the BCY10 and BCY25 electrolytes under fuel cell conditions, i.e., 5% hydrogen in argon at the anode with air at the cathode. The performance of BCY-based SOFC with Pt electrodes was measured over a temperature range of 500-800 degrees C using a galvanostat/potentiostat. Both anode and cathode product stream compositions were analysed using a gas chromatograph and a hygrometer. A material balance for the system was performed in order to estimate proton and oxygen ion fluxes across the electrolyte. Water transport across the electrolyte in the absence of an external circuit was also studied. (C) 2007 Elsevier B.V. All rights reserved.