In multilayer systems consisting of an ionic conductor and an electric insulator the ionic current can flow both across the bulk phase and along the heterophase boundaries. Here we report the results of a study on CSZ (ZrO2+8.7 mol.% CaO)/Al2O3 multilayer systems, representing a system with incoherent interfaces as shown by HRTEM (high resolution transmission electron microscopy). In order to separate the interface contribution of the total conductivity from the bulk contribution, the thickness of the CSZ and Al2O3 layers have been varied systematically, and the oxygen ion conductivity was measured parallel to the interfaces as a function of temperature. The total conductivity of the CSZ increases by two orders of magnitude when the thickness of the individual CSZ layers is decreased from 0.78 mu m to 40 run. It depends linearly on the reciprocal thickness of the individual layers, i.e. on the number of CSZ/Al2O3 interfaces, indicating a parallel connection between individual conduction paths in the bulk and the interfacial regions. The activation energy for the ionic conductivity measured in the temperature range between 350 and 700 degrees C decreases from about 146 kJ mol(-1) to 104 kJ mol(-1) by reducing the CSZ layer thickness. The activation energy for the interfacial transport is evaluated as 70 kJ mol(-1), suggesting a much higher ionic mobility in the disordered core regions of incoherent interfaces than in the bulk. (c) 2007 Elsevier B.V. All rights reserved.