Composite ionic conductors were produced by combination of alkaline carbonates with various ceramic oxides, including ceria, zirconia and alumina. The adopted mechano-thermal processing routes originated materials with the same nominal composition but with significantly different microstructures. The electrical performance of these composites, studied by impedance spectroscopy, showed the relevance of the mixed carbonate phase on the transport properties, but also unexpected composite effects. The role of grain size on the electrical performance could not be isolated from the possible influence of alkaline metal rich interfacial layers (ceramic/carbonate) consisting of nanosized 1D crystals. Modest electrode impedances in open air experiments are coherent with the coexistence of various charge carriers. The known interaction of major composite constituents and minor secondary phases with water might explain the presence of additional charge carriers. Copyright (C) 2011 John Wiley & Sons, Ltd.