The effects of cobalt addition (0.5 and 1 wt.%) on densification and ionic conductivity of Ce0.9Sm0.1O1.95 (10SDC) and Ce0.9Sm0.075Y0.025O1.95 (2.5Y-SDC) have been studied. X-ray diffraction (XRD) showed that Co had changed to Co3O4 and Co3O4 + CoO after firing at 900 degrees C and 1300 degrees C respectively. The addition of Co promoted densification to occur at lower temperatures with a more uniform grain growth and greatly improved both grain boundary and bulk conductivity for 10SDC. Significant improvement of grain boundary for the 2.5Y-SDC samples was obtained, even at 1300 degrees C sintering, while bulk conductivity was slightly improved. Rapid grain growth along with improvement of ionic conductivity was observed when the samples were sintered further at higher temperature. Superior ionic conductivity of the 2.5Y-SDC samples with Co addition to that of the bare 10SDC suggested the potential use of Co as the co-dopant in this system to reduce the content of costly rare earth usage. (C) 2009 Elsevier B.V. All rights reserved.