The characteristics of novel BaZr0.4Ce0.4In0.2O3 (BZCI) proton conducting ceramics and their application to hydrogen sensors were investigated. Their properties were examined by the EMF responses of a hydrogen concentration cell and an electrochemical hydrogen permeation test. The BZCI ceramic exhibited good proton conduction in an atmosphere containing hydrogen in the temperature range of 300-700 degrees C. To verify that these ceramics are practical protonic conductors, limiting-current-type hydrogen sensors using BZCI ceramics as electrolytes were constructed. They exhibited good sensing properties in a reducing atmosphere and hardly degraded on repeated use. BZCI seems to be a very useful protonic conductor in terms of reliability and conduction. (c) 2005 Elsevier B.V. All rights reserved.