Amorphous silicon oxynitride thin films were deposited on 4 inch (100) Si wafers in a conventional low pressure chemical vapor deposition reactor at 812 degreesC and 230 mTorr, using SiCl2H2-NH3-N2O gas mixtures. Depositions were carried out keeping the flows of SiCl2H2, and of NH3 constants at 20 and 60 seem, respectively, while that of NO was varying between 0 and 180 seem. The optical properties of deposits were studied with the aid of the Forouhi-Bloomer model (Phys. Rev. B34, 7018 (1986)) for amorphous semiconductors. From the analysis of the optical spectra the complex refractive index of the films as well as their thicknesses were derived. Moreover, an approximate picture of the energetic distribution of the density of electronic states near the absorption threshold of these films was obtained. This picture was found to be consistent with theoretical calculations found in the literature for crystalline silicon nitride and oxynitride. Thus, the increase of oxygen content in films does not cause a shift of valence and conduction band towards lower and higher energies, respectively, but it implies rather an increase of the band gap and a decrease of the energetic distance between their centers of gravity due to the appearance of a local maximum near the top of the valence band. This maximum is due to the additive effect of the N 2p and the O 2p non-bonding orbitals. The study of optical properties provides also useful information regarding the presence of electronic states within the forbidden gap due to film defects, coherent with electrical measurements. (C) 2003 Elsevier Science B.V. All rights reserved.