The ionic conductivities of cerium oxide doped with oxides of the entire rare earth series (except Pm) were measured using ac impedance in air at temperatures ranging from 400 degrees C to 900 degrees C. All of these samples showed an increase in the conductivity over undoped ceria, with ceria doped with Gd, Sm and Y exhibiting the highest values. The maximum conductivity observed (at 600 degrees C) was 1.2 X 10(-2) S cm(-1) for Ce0.8Sm0.2O1.9. The conductivity of yttria-doped ceria varied substantially with the dopant level. Maximum conductivity was observed for the composition Ce1.84Y0.16O1.92 Although the conductivity generally increased with increasing ionic radius of the dopant cation, no firm correlation was found between these two variables. For several of the materials, the relative contribution of the bulk crystal lattice and grain boundaries to the overall resistance was measured; the relative significance of each depended on the specific rare earth dopant and the processing conditions.