Composite ceramics based on magnesium oxide (MgO) as insulator (Ins) with gadolinium doped ceria (GDC) as solid electrolyte (SE) were prepared for model verification. It was found that SE/Ins grain boundaries exhibit conductivities 4-8 times higher than those of SE single phase material despite the fact that in this work the grain sizes of these composite ceramics were not small enough for the formation of a continuous network of the SE/Ins grain boundaries. This result indicates that the proposed model and strategy to manufacture composite materials with significantly higher ion conductivities can be used successfully, if an appropriate technology for establishing nanodispersions of homogeneously distributed components is available. (C) 2016 Elsevier B.V. All rights reserved.