Rhodium(III) ion (Rh 3+)-modified TiO 2 (Rh 3+/TiO 2) samples having various physical properties and structures (anatase and rutile) were prepared using commercially available TiO 2 samples. Thus-prepared Rh 3+/TiO 2 samples were used for degradation of acetaldehyde or mineralization of acetone with a constant vapor pressure under irradiation of visible light and correlations between the photocatalytic activities and physical properties were investigated. Rhodium(III) ion on the surface of TiO 2 worked as an inorganic photosensitizer and the Rh 3+/TiO 2 samples showed a kind of volcano-like tendency against specific surface area of TiO 2, indicating that the balance of surface area and crystallinity of TiO 2 is important in an Rh 3+/TiO 2 photocatalyst. The latter reaction system was found to be a simple and convenient method to evaluate the activity of a photocatalyst in a short time. The role of Rh 3+ as a catalyst for multi-electron reduction of oxygen is discussed on the basis of results obtained for Rh 3+-modified rutile-type TiO 2 samples.