In this paper we report on the effect of annealing on the microsctructural and optoelectronic properties of titanium dioxide (TiO2) thin films prepared using solegel method onto silicon (Si) (100) and quartz substrates. The annealing temperatures range from 200 to 1000 degrees C. The Microstructural properties of annealed thin films were investigated by Thermal gravimetric analyses (TGA), X-ray diffraction (XRD) and Raman Spectroscopy. The surface morphology of the film was examined using Atomic Force Microscopy (AFM) method. The optical properties of TiO2 thin films were characterized using UV-VIS and Spectroscopic ellipsometry. The results have shown that the TiO2 thin films persist in the anatase phase even after annealing at 800 degrees C. The phase transformation from anatase to rutile occurred only when the films were annealed at 1000 degrees C. AFM studies revealed nanocrystalline structure where their shape and density depend strongly on the annealing temperatures. The elaborated nanostructured-TiO2 thin films present a high transparency in the visible range. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO2 thin films. Spectroscopic ellipsometry and UV-VIS shows that the band gap of TiO2 thin films was found to decrease when the annealing temperature increases. The Anatase phase was find to show higher photocatalytic activity than the rutile one. (C) 2011 Elsevier B. V. All rights reserved.