Zr-substituted BaCeO3 (BaCe0.9-xZrxY0.1O3-alpha) was synthesized and its electrical conduction behavior and chemical stability were investigated. Single phases were confirmed over the whole range of x values (0.0 less than or equal to x less than or equal to 0.9). These oxides exhibited pure protonic conduction in hydrogen-containing atmosphere while they showed protonic, oxide ionic and electronic mixed conduction under high oxygen partial pressure at elevated temperatures. The chemical stability against CO, increased with an increase in zirconium content, although the protonic conductivity decreased with increasing x. (C) 2000 Published by Elsevier Science B.V.