Inhomogeneous microstructures, including nanosized domains and nanoprecipitates, were observed in sintered samples of holmium-doped ceria. The formation of such microstructural inhomogeneity is related to the segregation of dopant cations and can be enhanced by increasing doping concentration and sintering temperature. Through examination of the relationship between the microstructures and electrical properties, it was found that the inhomogeneity can have negative impacts on ionic conduction, which could be explained in terms of the interaction between the oxygen vacancies and the segregated dopant cations. (C) 2007 The Electrochemical Society.