Porous samples of Sm2O3-doped CeO2 (samaria-doped ceria, SDC) of composition Sm0.15Ce0.85O2-delta were made by conventional ceramic processing and sintering in air at 1400 degrees C. Crystal structure and microstructure of the samples were characterized, respectively, by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrical conductivity was measured using a four probe DC method over a temperature range from 200 degrees C to 800 degrees C, and over a wide range of oxygen partial pressures corresponding to testing in oxygen and in nearly dry hydrogen. Conductivity rapidly stabilized at any given temperature consistent with the attainment of thermodynamic equilibrium corresponding to the imposed conditions. At and below 300 degrees C, the conduction was predominantly due to oxygen ion transport. At and above 400 degrees C, however, significant electronic conduction occurred in reducing atmospheres. The ionic transference number of SDC at 400 degrees C in hydrogen is only similar to 40.4. This result shows that the electrolytic domain of SDC at and above 400 degrees C is rather narrow. These results also suggest that SDC (and possibly other rare earth oxide-doped CeO2) is not a suitable electrolyte without a thin electron blocking layer such as yttria-stabilized zirconia (YSZ). (C) 2011 Elsevier B.V. All rights reserved.