Single-crystal In2O3 and Sn-In2O3 (2.6% Sn) were grown by the flux method for use in an oxygen diffusion experiment. Diffusion profiles within the crystals were analyzed by secondary ion mass spectrometry. The concentration profile of O-18 in the solids could be explained very well by application of Fick＇s law The oxygen diffusion coefficients in Sn-In2O3 were higher than those in In2O3. However, the apparent activation energy for oxygen diffusion in Sn-In2O3 was smaller than that in In2O3, probably due to the occurrence of a nonstoichiometric reaction in this oxide. Oxygen diffusion in single-crystal In2O3 was slower than that reported for polycrystalline In2O3, although the activation energy was similar for both materials.