Spectroscopic ellipsometry is the technique of choice to characterize thickness and refractive indices of transparent and semi-transparent thin layers with thickness ranging from few Angstroms to few micrometers. However, in case of porous thin film, traditional EMA (Effective Medium Approximation) fails to provide qualitative information on porosity. However, Spectroscopic Ellipsometry can benefit elaborated hardware and software set-up involving the adsorption/desorption of an adsorbate in the porous thin film, we call it, Ellipsometry Porosimetry Atmospheric (EPA) and the adsorbate is water. The change in refractive index induced by the introduction of water is measured and thanks to Lorentz Lorenz effective medium model, the volume of water adsorbed by the material is calculated. EPA becomes an effective method for characterization of porosity volume, pore size distribution (PSD), average pore size, cumulative surface area and Young’s modulus of porous films. EPA is also suitable to evaluate the sealing of a porous layer. An example of study is carried on for TiO2 mesoporous prepared by Evaporation Induced Self-Assembly with various calcinations treatment. Plots results are characteristic of mesoporous medium with well defined pore dimensions. Upon calcinations, EP-A allows to follow a significant pores enlargement from 3 to 10 nm. In this paper, the instrument as well as TiO2 porous thin film study will be presented.