Rhodium(III) ion (Rh3+)-modified TiO2 (Rh3+/TiO2) samples having various physical properties and structures (anatase and rutile) were prepared using commercially available TiO2 samples. Thus-prepared Rh3+/TiO2 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 TiO2 worked as an inorganic photosensitizer and the Rh3+/TiO2 samples showed a kind of volcano-like tendency against specific surface area of TiO2, indicating that the balance of surface area and crystallinity of TiO2 is important in an Rh3+/TiO2 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 Rh3+ as a catalyst for multi-electron reduction of oxygen is discussed on the basis of results obtained for Rh3+-modified rutile-type TiO2 samples. (C) 2010 Elsevier B. V. All rights reserved.