We report evidence of photoisomerization-induced changes in the optical thickness of molecularly thin (9 Angstrom) photochromic layers which consist of azosilane molecules immobilized on silicon oxide substrates. Further, this light-induced change in the optical thickness of these Rims can be reversed upon irradiation with light Of an appropriate wavelength, This has been demonstrated by means of surface plasmon spectroscopy that allows the observation of very small changes in the optical thickness of ultrathin layers. The study of the thermal back isomerization reaction of the azobenzene molecules in these films reveals steric hindrance at the molecular level, as shown by using UV-vis spectroscopy.