We present the optical properties of TiO 2 deposited, by sol–gel method, on porous n+type GaAs substrates with different porosities and were thermally annealed at 600 °C for 15 min. The surface topography and density of grains packing at different samples were determined based on AFM images. AFM studies showed that the structure of the TiO 2 thin film was nanocrystalline with grains size dependent on the etching time and ranging between 4 and 7 nm. The optical constants ( n and k) of the films, as a function of etching time of GaAs substrate before and after deposition and content, were obtained using spectroscopic ellipsometer with rotating polarizer type in the ultraviolet–visible–near infrared (UV–vis–NIR) regions. The thickness of TiO 2 thin films (top layer) and the mixed layer of TiO 2 and porous GaAs (bottom layer) were determined by scanning electroscopic spectroscopy (SEM) and confirmed by spectroscopic ellipsometry using a modified Cauchy dispersion law for optical index calculation and through an optical model with double layers dispersion, we were able to determine the change of the optical properties of porous GaAs with integration effect of the TiO 2 particle.