Ionic-electronic mixed conductors consisting of Bi2CuO4 (electronic conductive matrix phase) and Bi2O3 (ionic conductive phase) have been investigated. It is shown that electrical conductivity and oxygen ion transference number of Bi2CuO4+(5, 10, 15, 20 wt.%) Bi2O3 ceramic composites are conditioned by the grain boundary wetting (by eutectic liquid) transition and substantially depend on average grain size of Bi2CuO4 and content of Bi2O3. (C) 2003 Elsevier B.V. All rights reserved.