The TiO2 powder (Degussa, P-25), which is a standard material in the field of photocatalytic reactions, contains anatase and rutile phases in a ratio of about 3 : 1. Transmission electron microscopy showed that the anatase and rutile particles separately form their agglomerates. The average sizes of the anatase and rutile elementary particles are 85 and 25 mn, respectively. Diffuse reflectance spectra of the TiO2 powder were successfully traced by physically mixing pure anatase and rutile particles in a ratio of 3: 1. By the BF treatment of the TiO2 powder, pure rutile particles were isolated. All these results indicate that the rutile phase does not exist as an overlayer on the surface of anatase particles, but it exists separately from anatase particles. We also found that photocatalytic oxidation of naphthalene is inefficient on pure anatase and rutile powders. However, the reaction is very efficient on the P-25 powder, as well as on a mixture of pure anatase and rutile particles. Under the conditions of the photocatalytic reactions, the anatase and rutile agglomerates are considered to be decomposed, and the anatase and rutile particles are in contact, leading to a synergy effect.