Abstract This paper is focused on nanoporous methylsilsesquioxane deposited using a polymer approach and shows the complementarities of three experimental techniques: ellipsometric porosimetry (EP), X-ray reflectivity (XRR), and grazing incidence small-angle X-ray scattering (GISAXS). XRR and EP confirm that the pore volume fraction is larger for smaller dielectric constants. EP and GISAXS find mean pore sizes independent of the porosity, in the range 3–4 nm as diameter. GISAXS is the only technique that can estimate the porosity isotropy. Finally, the impact of integration processes such as surface plasma treatment, etching or stripping on the porosity is evaluated: the porosity remains unchanged except in the superficial layer where an increase of the pore size (or of the roughness) is observed.