Spectroscopic ellipsometry (SE) was used to characterize Ba0.7Sr0.3TiO3 (BST) thin films prepared by the sol-gel method. Ten samples were investigated, each consisting of a sol-gel BST thin film on Si substrate and annealed at 400, 500, 600, 700, and 800 degrees C, respectively, in O-2 and N-2 ambients. Our main objective was to determine the optical properties of the BST thin films as a function of annealing temperature for annealing in the two chosen ambients, In the analysis of the measured SE spectra, a Cauchy dispersion function was used to represent the optical properties of the BST thin films. For each sample studied, good agreement was obtained between the measured spectra and the model calculations in the chosen spectral region. Our analysis clearly shows that the refractive index of the BST films annealed in both ambients increases as the annealing temperature increases, with the index value of the films annealed in the O-2 ambient increasing more dramatically at temperatures above 600 degrees C, which, we believe, is an indication of further crystallization of the films. We confirmed this phenomenon via X-ray analysis of the films.