SiOx and SiNx films with varied stoichiometries were obtained by low-pressure chemical vapor deposition at low temperature from Si2H6/N2O and Si2H6/NH3, gas mixtures respectively. Using the Clausius-Mossoti theory and the Bruggeman expression, a relation between the refractive index and the stoichiometry of these two materials is established and corroborated by experiments thanks to X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electron energy loss spectroscopy or/and the differential thickness method. These data have also been correlated in order to calculate the electronic polarizability of silicon atoms into SiOx and SiNx materials.