This work presents the annealing temperature effect on the properties of mercury (Hg)-doped titanium dioxide (TiO2). Thin films and polycrystalline powders have been prepared by sol-gel process. The structure, surface morphology and optical properties, as a function of the annealing temperature, have been studied by atomic force microscopy (AFM), Raman, reflectance and ellipsometric spectroscopies. In order to determine the transformation points, we have analyzed the xerogel-obtained powder by differential scanning calorimetry (DSC). Raman spectroscopy shows the crystalline anatase and rutile phases for the films annealed at 400 degrees C and 1000 degrees C respectively. The AFM surface morphology results indicate that the particle size increases from 14 to 57 nm by increasing the annealing temperature. The complex index and the optical band gap (E-g) of the films were determined by the spectroscopic ellipsometry analysis. We have found that the optical band gap decreases by increasing the annealing temperature. (C) 2011 Elsevier B. V. All rights reserved.