N-doping is often used to improve the photocatalytic properties of TiO2, films in order to achieve visible light response. In this work, we study the effect of annealing treatment (temperature and atmosphere) on the structural and optical properties of undoped and N-doped TiO2, films by means of X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Rutherford backscattering (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) and UV-VIS optical transmission spectroscopy. Porous five-layer TiO2, films were deposited by sol-gel method on quartz substrate and thermally treated in oxygen or NH3 flow at 500 and 600 degrees C. Significant doping effect was achieved after annealing at 600 degrees C in NH3 and a shift in optical bandgap value down to visible range (2.69 eV) was observed. (C) 2008 Elsevier B.V. All rights reserved.