Silicon oxynitride films exhibiting refractive indexes in the 2.01–1.49 range have been deposited in a plasma assisted chemical vapor deposition system using SiH4+NH3+O2 gas mixtures. The O2/NH3 gas ratio was varied in order to obtain different oxynitride compositions, ranging from silicon nitride to nearly stoichiometric silicon oxide. A single peak, in contrast to two separate peaks normally associated with silicon oxide and silicon nitride, was observed in the IR spectra, indicating the formation of a unique oxynitride compound. The IR absorption spectra as well as the refractive indexes measured by ellipsometry were used to estimate the stoichiometry of the films, the results being well correlated to Auger analysis.