The multiple-angle-of-incidence ellipsometric studies of the natural film formed on the surface of porous silicon sample after its production and subsequent keeping in isopropyl alcohol have been performed. A new property of porous silicon with the natural film on its surface has been revealed which consists in a drastic change of the polarization characteristics of the reflected light wave after its keeping in alcohol. This property of porous silicon may be the basis for the development of liquid and gas sensors, polarization elements of optical devices etc. It was found that ellipsometric curves are described better by calculations in a two-layer model in comparison with a single-layer one of the investigated film. It was obtained that the outer layer of the film has smaller refractive index than the inner one. As a result of decoration the refractive indices of both layers decrease and further the changes of refractive indices and the redistribution of the thickness of both layers are found. It is suggested that the interaction of the alcohol molecules with silicon and its oxide environment occurs in the walls of the pores during the decoration.