The present work aims at the investigation of Ln(0.2)Ce(0.8)O(2-delta) (where Ln = Sm, La, Gd) structural and electrical properties when in the Ln sub-lattice, Ba2+ and Sr2+ with ionic radii 1.42 and 1.26 angstrom, respectively, are introduced. The conductivity measurements were held both in air and in H-2 + 3%H2O atmosphere using the 4-probe dc technique at the temperature range of 600-900 degrees C. Among all the samples, the highest value of electrical conductivity is obtained in the case Of (Sm0.75Sr0.2Ba0.05)(0.2)Ce0.8O2-delta, both in air and in hydrogen atmosphere. In the case of H-2 + 3%H2O the conductivity of the co-doped compounds increases in comparison with air. Moreover, the dependence of conductivity on the oxygen partial pressure, measured at the P-O2 range of 0.21-10(-22) atm, showed that the electrolytic area of alkaline-earth metals doped Ln(0.2)Ce(0.8)O(2-delta) is considerably enhanced, as predicted by the theory. Finally, by comparing the thermal expansion coefficients of the different materials (TEC), the thermo-mechanical compatibility between the co-doped and the other cell components was also investigated. (c) 2008 Elsevier B.V. All rights reserved.