Oxide thin films synthesized using the sol-gel technique have the advantages of low cost, high thickness control, tunable refractive index and silicon technology compatibility, properties that make them potential materials for optoelectronic applications. For very thin films with low porosity, the determination of sorption and optical properties is quite complex because of the small sample size. Thus, there is a need to use especially designed techniques to obtain reliable results. In this work, a comprehensive study on the porosity evolution of SiO2 and TiO2 thin films using X-Ray Reflectometry and Environmental Ellipsometric Porosimetry is presented. For sol-gel SiO2 thin films, it was found that the effective refractive index increases with thermal treatment as the porosity decreases. However, the refractive index of the walls was found constant. For sol-gel TiO2 films, crystallized in anatase phase, both the effective refractive index and the wall refractive index increase with thermal treatment. SiO2 and TiO2 thermal oxides were also characterized for comparison. (c) 2012 Elsevier B.V. All rights reserved.