Ceria-based materials are prospective electrolytes for intermediate-temperature solid oxide fuel cells. The ionic conductivities of ceria-doped with Sm, Gd, Dy and Er are investigated as a function of temperature by using a.c. impedance. The results show that conductivity depends on the rare earth dopant, its amount, an appearance of second phase, and the microstructure. With 10 mol% dopant, Sm exhibits higher conductivity than Gd, Dy and Er, respectively. With an increase in Dy content, the total conductivity increases, which is attributed to an increase in grain boundary conductivity. By contrast, an increasing amount of Er from 10 to 20 mol% reduces the conductivity of ceria and results in a separated phase of Er2O3 as detected by X-ray diffraction and scanning electron microscopy. In addition, the grain size corresponding to grain boundary density affects the conductivity due to the contributions from the grain interior and grain boundary conductivities. (C) 2007 Elsevier B.V. All rights reserved.