The electrical conductivity and ionic domain of several ceria-doped yttria compositions, with up to 10 cation% dopant, were studied as a function of temperature (800-degrees to 1100-degrees-C) and oxygen partial pressure (10(-15) to 10(5) Pa). The ionic conduction in ceria-doped yttria involves the transport of interstitial oxygen ion defects. The increase of electronic conductivity under reducing conditions with increasing dopant concentration suggests hopping of small polarons between cerium ions with different valences as the dominant conduction mechanism.