Oxidation of 2-propanol to acetone on a Pt/TiO<Sub>2</Sub> photocatalyst by UV pulse irradiation was observed by time-resolved IR spectroscopy (TR-IR). The hole transfer to the 2-propanol, which triggers the oxidation, was completed within 0.5 μs. Transient vibrational spectra showed a new band at 1640 cm<Sup>−1</Sup>, which appeared immediately after the irradiation. This band gradually transformed to a band at 1700 cm<Sup>−1</Sup>, which was assigned to the C=O stretching mode of acetone, the product of 2-propanol oxidation. Thus, the band at 1640 cm<Sup>−1</Sup> was suggested to be the reaction intermediate that transforms to acetone. We discussed the origin of this band using density functional calculation, and an acceptable agreement was obtained with the anion radical of acetone adsorbed on the catalyst.