Transparent TiO2 thin film photocatalysts were prepared on transparent porous Vycor glass (PVG) by an ionized cluster beam (ICB) method. The UV-VIS absorption spectra of these films show specific interference fringes, indicating that uniform and transparent TiO2 thin films are formed. The results of XRD measurements indicate that these TiO2 thin films consist of both anatase and rutile structures. UV light (lambda > 270 nm) irradiation of these TiO2 thin films in the presence of NO led to the photocatalytic decomposition of NO into N-2, O-2 and N2O. The reactivity of these TiO2 thin films for the photocatalytic decomposition of NO is strongly dependent on the film thickness, i.e., the thinner the TiO2 thin films, the higher the reactivity.