Monodispersed, agglomerate-free In2O3 hollow spheres have been prepared via a simple synthetic route involving permeation and anchoring of In3+ ions with carbonyl groups of swollen commercial polymer beads in tetrachloroethylene solvent followed by thermal removal of the template cores in ambient air. The as-synthesized hollow spheres exhibit a narrow size distribution with tunable particle size (0.5–1.2 μm) and shell thickness (62–230 nm) over the process variables examined, i.e., InCl3 precursor concentration (4.5 × 10−3–6.7 × 10−2 M), reaction temperature (55°C–95°C), and reaction time (1–6 h). Kinetics calculation reveals that the formation of permeating In3+-rich shell in the swollen template beads becomes energetically less favorable to proceed as the reaction time increases. This limits the maximum shell thickness attainable at the given process variables. The shell is nanoporous with a Horvath-Kawazoe (HK) pore size of ~3 nm, which remains essentially unchanged as the process variables alter. The In2O3 hollow spheres with an increased Brunauer-Emmett-Teller (BET) surface area (up to 329 m2/g) show an improved capability in photodegradation of aqueous methylene blue (MB) dye under UV exposure as well as an increased sensitivity for CO-gas detection. This metal-implantation scheme is general and can be extended to the synthesis of other hollow materials in various solvent liquids.