HRTEM measurements of silicon rich oxides (SRO) show silicon microcrystals in an oxide matrix . Simple, reliable characterization of this two phase material has been a problem. Ellipsometric measurement of the refractive index is a convenient method for characterizing SRO films. Film composition can be related to the refractive index by Bruggeman's effective medium approximation. In this paper we demonstrate correlation of film compositions obtained by this technique with those obtained by Auger electron spectroscopy (AES)and Rutherford back scattering (RBS). We further demonstrate regimes of LPCVD growth where simple correlation of film composition with [N2 0]/[SiH4] gas ratios is not reliable.