We have found that anatase TiO 2 dissolves into an HF solution more easily than rutile TiO 2 . Using this property, we succeeded in isolating pure rutile particles from TiO 2 powder consisting of rutile and anatase phases by the HF treatment. The isolation of pure rutile particles was confirmed by X-ray diffraction patterns and diffuse reflection spectra. The rutile particle showed much lower photocatalytic activity than the original powder, if compared for the oxidation of 2-propanol in aqueous solution using dissolved oxygen as the electron acceptor. On the other hand, when Fe(III) ions were used as the electron acceptor for oxidation of 2-propanol and water, the activity scarcely changed after the HF treatment. These results indicate that rutile TiO 2 has poor activity for the reduction of oxygen, and the presence of a small amount of an anatase component is crucial for an efficient photocatalytic reaction on TiO 2 particles using oxygen as the electron acceptor.