Composite electrolytes based on Gd-doped ceria (from 50 to 95 vol%) and one eutectic mixture of sodium and lithium carbonates were prepared by co-firing, in order to identify the compositional range where both oxide and carbonate-ion transport are balanced, as required for high performance CO2 separation membranes. Considering the adequacy of this technique, the effects of composition and microstructure on oxide-ion transport were inspected by impedance spectroscopy. The specific features of low temperature impedance spectra allowed access to data on oxide-ion transport based on a hereby suggested equivalent circuit where the individual characteristics of both constituent phases are considered, as well as the variable nature and relevance of interfaces. Combined analysis of all information indicates that the range of interesting compositions for balanced dual ionic conduction corresponds to a ceramic phase content around 85 yo1%, for the present microstructural characteristics and a target temperature below 550 degrees C. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.