A plausible reaction mechanism for a visible light photocatalyst of TiO2 modified with platinum(IV) chloride (PtCl) was proposed on the basis of the measurements with electron spin resonance (ESR) spectroscopy and chemiluminescence photometry. Under visible light (lambda > 500 nm) irradiation, the deposited Pt(IV) chloride is charge-separated into Pt3+ and Cl radical by the excitation of the ligand-to-metal charge transfer. The Pt3+ gives an electron to the conduction band of TiO2, which has Pt3+ return to Pt4+. The electron in the conduction band reduces the oxygen molecule into O-2(-). The presence of Pt3+ and O-2(-) has been elucidated in the present study. Moreover, valence band holes of TiO2 were detected by ESR spectroscopy under visible light irradiation. Therefore, besides being used to oxidize organic compounds, the photogenerated Cl radicals likely receive electrons from the TiO2 valence band by visible light excitation, producing the valence band holes. Because the valence band holes have a stronger oxidation power than Cl radicals, the excitation of valence band electrons to Cl radicals would be the origin of the high photocatalytic activity of the PtCl-modified TiO2 under visible light irradiation.