The effects of various contents of alumina co-doping on a ceria-based electrolyte were investigated. All samples were sintered at 1500 degrees C for 10 h. The X-ray diffraction (XRD) patterns showed a fluorite cubic structure. To investigate the surfaces of the Ce-0.8(Sm0.5-xY0.5Alx)(0.2)O2-delta (CSYA) ceramics by scanning electron microscopy (SEM), the electrical properties of the CSYA ceramics were measured by AC impedance. The optimal values of ionic conductivity and activation energy were reached at x=0.4 measured at 600 degrees C. This result indicated that the alumina co-doping improved the electrical and microstructural properties of the ceria-based electrolyte. (C) 2014 Elsevier Ltd. All rights reserved.