Thin and very uniform silica-alumina films mere deposited by dip-coating onto AISI-303 stainless steel and submitted to different heat treatments. They were prepared by simultaneous ultrasound-assisted hydrolysis and polycondensation of tetraethylorthosilicate and aluminium-sec-butylate or aluminium nitrate in an atmosphere rich in water vapour. The resulting materials were characterised by ellipsometry in order to optimise the process of avoiding porosity which was submitted to molecular probing. This method informs us of the micropore size distribution, using adsorptive different-sized molecules. The thickness ranges from 60 to 350 nm, depending on the sol processing and heat treatment. In some samples most of the micropores are only sensitive to molecular probe below 0.7 nm diameter. These and other characteristics will be discussed.