Sputter deposition of indium (In) in ionic liquids (ILs) could produce stable In metal nanoparticles whose surface was covered by an amorphous In2O3 layer to form In/In2O3 core/shell particles. The size of the In core was tunable from ca. 8 to 20 nm by selecting the kind of IL, whereas the shell thickness of In2O3 was almost constant at ca. 1.9 nm. Heat treatment of the thus-obtained particles at 523 K in air oxidized In metal of the core, resulting in the formation of spherical hollow particles made of crystalline In2O3. The size of the hollow particles was slightly larger than that of the In/In2O3 core/shell particles used as a starting material, whereas the void space formed inside hollow particles was smaller than the corresponding In metal cores. These facts indicated that in addition to the predominant outward diffusion of In ions, an inward transport of oxygen ions occurred, and thus an In2O3 crystal could be grown on both the inner concave and outer convex surfaces of the oxide shell layer.