Composites based on Gd-doped ceria and alkaline carbonates were produced in different manners to identify key compositional and microstructural features determining their mixed oxide and carbonate-ion conductivity. Co-firing of fine powder precursors yielded composites with delicate ceramic skeletons. After impregnation with alkaline carbonates, pre-sintered ceramic substrates produced from coarsened ceramic powders showed better densification and performance with respect to oxide-ion conduction. Microstructural analysis combined with low temperature impedance data obtained with ceramic substrates and composites confirmed the potential of impedance spectroscopy to infer on the oxide phase contribution to charge transport. (C) 2013 Elsevier B.V. All rights reserved.