Atomic Layer Deposition has been used to deposit titanium dioxide thin films on soda-lime glass substrates. A series of films with thicknesses from 2.6 to 260 nm has been created and the film structure has been studied with X-ray diffraction. It has been observed that at a reaction temperature of 350 degrees C, titanium dioxide thin films initially grow as anatase but after a certain thickness, growth continues as rutile. The photoactivity and photocatalytic activity of the films have been found to reach their maximum at a film thickness of 15 nm. At this thickness, the film structure shows a small fraction of rutile crystallites in a largely anatase matrix indicating that both crystal phases are necessary for the maximum activity. (C) 2009 Elsevier B.V. All rights reserved.