To study the effects of ions diffused from the glass substrate on TiO2 photocatalysis, TiO2 thin films were prepared on different kinds of glass substrates at different heating temperatures. The TiO2 films were characterized using X-ray photoemission spectroscopy (XPS), X-ray diffraction (XRD), ellipsometry, and absorption spectroscopy. The photocatalytic activities of the TiO2 thin films were evaluated using the degradation reaction of trichloroethylene (TCE). Diffused ions such as Na+ and Si4+ raised the anatase formation temperature and increased the particle size of TiO2 thin films (r c). The photocatalytic activity of the thin films largely depended on the particle size of TiO2; the reciprocal number of the half-life of the TCE degradation, t 1/2 -1, was roughly proportional to r c -1.8. The conventional recombination center hypothesis for the Na+ ions might not hold for the sol−gel TiO2 films.