The lattice and grain-boundary diffusion coefficients of ytterbium, which substitutes for yttrium, have been determined in high-purity, stoichiometric yttrium. aluminum garnet (YAG) polycrystals in the temperature range 1400 degrees -1550 degreesC, in air. Ytterbium oxide thin films were produced on the YAG surfaces by a dipping method. After diffusion treatments, the penetration profiles were established by secondary ion mass spectroscopy, and the diffusion coefficients were calculated from the thin-film solution of Fick's equation. The difference between the volume and grain-boundary diffusion coefficients is similar to5 orders of magnitude in the temperature range studied. The cation activation energies (similar to 550 kJ/mol) are much larger than those for oxygen (similar to 300-350 kJ/mol). The effective diffusion coefficient deduced from high-temperature deformation data reported in the literature for YAG polycrystals, assuming grain-boundary sliding accommodated by volume diffusion, is in excellent agreement, both in magnitude and activation energy, with the cation diffusion data.