Photoinduced oxidation of chemisorbed 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) in air at room temperature yielded a uniform SiOx monolayer with a thickness of approximately 0.2 nm on TiO2. The oxidation was suggested to be caused by the band gap excitation of TiO2 leading to the generation of activated oxygen species on the surface. The sequential cycles of chemisorption of TMCTS (process I) followed by photoirradiation (wavelength < 330 nm, process II) grew the SiOx (2 < x < 3) film 0.4 +/- 0.1 nm per cycle, whereas the rate of the oxidation exponentially decreased with an increase in the film thickness. The contact angle of H2O, being almost constant at 90 degrees +/- 3 degrees after each process I, decreased as the photoirradiation time increased during the subsequent process II. The gas phase adsorption experiments of pyridine further demonstrated that the surface acidic nature drastically changes from Lewis acid character to Bronsted acid character upon formation of the SiOx monolayer on TiO2. The photocatalytic activity was maintained to an appreciable extent in this state. Both the analyses of the diffuse reflectance infrared Fourier transform spectra and the data on the contact angle of H2O indicated that the surface oxidation obeys the Langmuir-Hinshelwood mechanism.