Transparent anatase TiO2 nanometer thin films with photocatalytic activity were prepared via the sol-gel method on soda-lime glass, fused quartz, and soda-lime glass precoated with a SiO2 layer. The resulting thin films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The photocatalytic activity of the various TiO2 films was evaluated by the photocatalytic decolorization of aqueous methyl orange. The diffusion of sodium and calcium ions from the soda-lime glass into the nascent TiO2 films was found to be detrimental to the photocatalytic activity of the resulting TiO2 films. Sodium and calcium diffusion into nascent TiO2 films was effectively retarded by a 0.3 mum SiO2 interfacial layer formed on the soda-lime glass.