A novel method for preparing SiOx (2 < x < 3) monolayers on TiO2, consisting of the chemical vapor surface modification with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS, process I) and subsequent ultraviolet photoirradiation (process II), is reported. X-ray photoelectron spectroscopy revealed that SiO, monolayers form layer-by-layer by cycling process I-II. The surface oxidation of the Si-H and Si-CH3 groups to Si-OH groups during process II was examined by difference diffuse reflectance Fourier-transformed infrared spectroscopic measurements. The monolayer of SiOx (similar to 0.2 nm, coverage similar to 0.86) drastically changed the acidic nature of the TiO2 surface, maintaining the photocatalytic activity, while multilayers (similar to 3 nm) reduced it greatly.