Sixteen different thermal profiles were used to exploit the role of thermal treatment on the structure, microstructure and electrical conductivity of yttrium- and gadolinium-doped ceria electrolytes. Samples were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy in air. The bulk and grain boundary conductivity dependence on grain size and thermal treatment deviated from usual trends. Consistent changes in the fluorite lattice parameter with thermal history, reported here for the first time, reveal a tendency for solid solution demixing. Formation of localized concentrations of dopant cations (both in bulk and grain boundaries), explains the apparently abnormal electrical performance of these materials after prolonged thermal treatments. (C) 2012 Elsevier B.V. All rights reserved.