[en] The relationship between the interdiffusion coefficient and the self-diffusion coefficients of constituent ions was theoretically derived for interdiffusion of the fluorite-cubic solid solution. The derived theoretical interdiffusion coefficient showed a good agreement with the experimentally determined interdiffusion coefficient for a fluorite-cubic ZrO2-CaO solid solution. The results confirmed that the interdiffusion was controlled predominantly by the diffusivity of the faster cation. (orig.)