Porous silicon (PS) has great potential in optical applications due to its tunable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indexes can be used as interference filters for applications in optoelectronics. In the present work, Rutherford backscattering spectroscopy (RBS) measurements and optical characterization have been carried out on PS multilayer stacks consisting of alternate low-porosity/high-porosity layers to determine their compositional profile, homogeneity, and overall optical behavior. In addition, RBS has been used for the first time to determine the porosity profile of this kind of structures. The experimental results show a constant indepth composition among alternate layers, revealing the good homogeneity of the multilayer structures. Neither porosity nor oxidation degree gradient were observed. (C) 2004 The Electrochemical Society.