[en] Recently-measured uranium concentration distributions in air-annealed mixed uranium-thorium oxides were analyzed by a model incorporating simultaneous direct volume diffusion and grain boundary diffusion. The diffusivities for these two processes were strongly affected by the uranium content of the mixed oxide; mobility increased with the cation fraction of uranium. This effect is attributed to the increase in hyperstoichiometry which accompanies replacement of thorium by uranium. Porosity which developed as a result of depleting the solid of uranium did not appear to have a significant effect on grain boundary diffusion. (orig.)