Obliquely evaporated films of silicon oxide, SiOx, widely used to align the nematic liquid crystals in electro‐optics devices, were investigated optically by attenuated total reflection on a pyramid with a rectangular base covered with a thin layer of gold. This configuration allowed us to make measurements in two perpendicular directions and to determine any in‐plane anisotropy. Reflectivity measurements versus angles of incidence exhibited minima associated with both resonant guided modes and surface plasmon oscillations (SPO). Analyses revealed that the obliquely deposited SiOxlayers were slightly birefringent and porous, the porosity being determined by the SPO shift when air was replaced by a liquid, hexane. Here the voids surrounding the columnar deposits of SiOxbecame filled with either air or hexane and, in spite of the heterogeneous structure of the films, effective indices of refraction for the SiOxlayer could be determined. Applying the Maxwell–Garnett theory for spherical particles embedded in a host medium (air and hexane), we found that the dielectric constant of our SiOxlayers were close to that of stoichiometric SiO2with a slight optical anisotropy.