BaxCe0.8Er0.2O3-x (x = 0.98-1.03) solid electrolytes were synthesized by high temperature solid-state reaction. X-ray diffraction (XRD) patterns showed that the specimens were a single perovskite-type orthorhombic phase. The conduction properties of the specimens were studied electrochemically in the temperature range of 600 - 1000 degreesC. The influence of nonstoichiometry in the specimens with x not equal 1 on conduction properties was investigated and compared with that in the specimen with x = 1. These three specimens showed a good protonic conduction under wet hydrogen, a mixed conduction of oxide-ion and electronic hole under dry higher oxygen partial pressure, and a mixed conduction of proton, oxide-ion and electronic hole under wet higher oxygen partial pressure. Both of the protonic and oxide-ionic conductivities increased with increasing barium content in the specimens under wet hydrogen and dry air, respectively. (C) 2003 Elsevier B.V. All rights reserved.