Compressive high-temperature deformation of BaCe1-xYxO3-y polycrystals as a function of yttrium content (x = 0.05, 0.1, 0.15, and 0.2) was conducted in controlled oxygen partial pressures of 0.1-2 X 10(4) Pa at 1200-1400 degrees C to determine deformation kinetics and the rate-controlling species for diffusional creep. Creep rate was independent of oxygen partial pressure and the steady-state creep rate increased with increasing Y content. The results can be explained on the basis of a grain-boundary-sliding creep mechanism in which the rate-controlling species are Ba or Ce interstitials. A simple point-defect model was used to describe the results.