Porous samples of Sm 2 O 3 -doped CeO 2 (samaria-doped ceria, SDC) of composition Sm 0.15 Ce 0.85 O 2-ฤฑ were made by conventional ceramic processing and sintering in air at 1400 โข 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 โข C to 800 โข 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 โข C, the conduction was predominantly due to oxygen ion transport. At and above 400 โข C, however, significant electronic conduction occurred in reducing atmospheres. The ionic transference number of SDC at 400 โข C in hydrogen is only โผ0.4. This result shows that the electrolytic domain of SDC at and above 400 โข C is rather narrow. These results also suggest that SDC (and possibly other rare earth oxide-doped CeO 2 ) is not a suitable electrolyte without a thin electron blocking layer such as yttria-stabilized zirconia (YSZ).