This study focuses on the photocatalytic decomposition of benzene on a TiO2 (Degussa P-25)thick film, at a high concentration, fixed on a Pyrex glass substrate using a TEOS (tetraethylorthosilicate) binder. Under UV radiation (365 nm) for 1 h, 100 ppm of benzene was completely decomposed in a photocatalytic batch system. The most effective photodecomposition of benzene was observed at a Ti/Si ratio (in a TEOS binder) of 0.67 in a TiO2 film. In UV-Vis spectra, the absorption band of TiO2 shifted to a shorter wavelength when a TEOS binder was used. The Si ion introduced from the TEOS binder proved to be better at increasing the photodegradation rate of benzene. Based on the relationship between the UV-Vis spectra and the benzene photodegradation, two models of the change of the TiO2 band gap are suggested. The first model presents electrons released from the valence band of TiO2 through UV radiation; these electrons become transferred into the valence band of SiO2 at lower energy states. The electrons are then readily transferred into the conduction band of TiO2. The second model presents SiO2 formed from the TEOS binder exposed to UV light. Here, the electrons in the valence band of SiO2 are transferred to the conduction band of SiO2, where upon they are readily moved to the conduction band of TiO2.