Compressive creep tests of BaCe0.8Y0.2O3-alpha polycrystals were conducted in Ar to determine deformation rate and calculate the diffusivity of the rate-controlling diffusing species for diffusional creep. At 1200-1450 degrees C, the steady-state strain rate ((epsilon) over dot) could be expressed as (epsilon) over dot = A sigma(1.1+/-0.1) exp[(-343+/-30 kJ/mole)/RT] where A is a constant, sigma is the steady-state stress, R is the universal gas constant, and T is the absolute temperature. The data and microstructural observations indicated that creep occurred by a grain-boundary-sliding mechanism, which was probably controlled by lattice diffusion of either Ba or Ce.