Niobium oxide films have been prepared by metal-organic chemical vapor deposition (MOCVD) in two different systems via thermal decomposition of Nb(OEt)5.A detailed study by exsitu variable-angle spectroscopic ellipsometry indicates a wavelength dependence of the refractive index that could be described by a Cauchy type dispersion formula and typical values of 1.9 to 2.1 at 632.8nm. The band gap of these films were at about 3.5 and 4eV for the indirect and direct transition. Preliminary results of in-situ ellipsometry at four different wavelengths in the visible and UV will be discusssed. The films were amorphous, extremely smooth and pinhole free and therefore they are useful candidates for interdiffusion barrier layers.