The Pechinni method (A) as well as hydrothermal treatment (B) of co-precipitated CeO2-based gels with NaOH solution were used to synthesise pure CeO2, and CeO2-based solid solutions with formula Ce1-xMxO2, Ce1-x(M0.5Ca0.5)(x)O-2 M = Gd, Sm for 0.15 < x < 0.3 nanopowders. The thermal evolution of CeO2-based precursors during heating them up to 1000 degrees C was monitored by thermal (TG. DTA) analysis and X-ray diffraction method. All nanopowders and samples sintered were found to be pure CeO2 or ceria-based solutions with fluorite-type structure. The microstructure of CeO2-based sintered samples at 1500 degrees C (A) or 1250 degrees C (B) was observed for 2 h under the scanning electron microscope. The electrical properties of singly Ce1-xMxO2 or doubly doped CeO2-based samples with formula Ce1-x(M0.5Ca0.5)(x)O-2. M = Gd, Sm, 0.15 < x < 0.30 were investigated by means of the ac impedance spectroscopy method throughout the temperature range of 600-800 degrees C. It has been stated that partial substitution of calcium by samarium or calcium by gadolinium in the Ce1-x(M0.5Ca0.5)(x)O-2, M = Gd, Sm solid solutions leads to ionic conductivity enhancement comparable with only samaria- or gadolina-doped ceria. The CeO2-based samples with small-grained microstructures obtained from powders synthesised by hydrothermal method exhibited better ionic conductivity than samples with the same composition obtained from powders synthesised by the Pechinii method. The stability of the electrolytic properties of selected co-doped ceria sinters in fuel gases (H-2, CH4) as well as exhaust gases from diesel engine was also investigated. The co-doped Ce-0.8(Sm0.5Ca0.5)(0.2)O-2 or Ce-0.85(Gd0.5Ca0.5)(0.15)O-2 dense samples would appear be to more adequate oxide electrolytes than Ce1-xMxO2, M = Sm, Gd and x = 0.15 or 0.2 for electrochemical devices operating at temperatures ranging from 600 to 700 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.