Structural and A.C. impedance analyses were conducted for various ceria-based composite systems. Structural studies showed that the ceria-carbonate composites are two-phase materials, where carbonates were often amorphous. Two phases of ceria and carbonates are mixed at different particle size levels depending on the preparation techniques, especially, employing the NANOCOFC (nanocomposites for advanced fuel cell technology) approach to prepare ceria-LiNaCO(3) nanocomposites. General observations from structural analyses are that different preparation techniques resulted in two-phase composite particles in different particle sizes varying from micrometer level to nano-level accompanying also different homogeneity. General observations from impedance analyses are that for the nanocomposites (particle size at nano-scale) more complex grain boundary interface effects are observed compared to that for samples with grains of the micrometer level, but nanocomposites showed enhanced conductivities at the low temperatures. Interfaces and interfacial conduction mechanism can be concluded for such conductivity enhancement. Crown Copyright (C) 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.