Strontium titanate thin films have been prepared at different oxygen pressures and subjected to different post-deposition annealing treatments. The films were deposited on Si substrates with a silica buffer layer. Infrared reflectance measurements were performed to determine the optical parameters, and the spectra were fitted using adequate dielectric function forms for the layers and the substrate. The as-deposited films were found to be amorphous. Their infrared spectra presented bands corresponding to highly damped vibrational modes. As the annealing time and temperature were increased, the strontium titanate layer optical parameters became closer to those corresponding to bulk SrTiO3 and a strong increase of the silica layer thickness was observed. The silica layer growth was attributed to the gradual oxidation of the Si substrate during the annealing that induced also a blueshift of the silica Si-O asymmetric stretching frequency. This frequency shift was associated to a decrease in the silicon oxide density as the progressive layer oxidation originated a variation of its stoichiometry from SiO1.5 towards amorphous SiO2. (c) 2006 Elsevier B.V. All rights reserved.