In this work, the correlation between BEMA model and FTIR analysis was employed to investigate the chemical composition of silicon oxynitride (SiOxNy) films deposited by low pressure chemical vapour deposition (LPCVD) technique at temperature of 850 0 C from nitrous oxide N2O, ammonia NH3 and dichlorosilane SiH2C12. Different stoichiometries were obtained for different values of relative gas flow ratio NH3/N2O while keeping the SiH2Cl2 flow constant. The optical properties were studied using spectroscopic ellipsometry. Apart from films thickness, their refractive index as well as their SiO2 and Si3N4 volume fractions were deduced using the Bruggeman effective medium approximation (BEMA) model. It was found that the refractive index increases from 1.45 to 1.60 with increasing nitrogen incorporation. The Fourier Transform Infrared spectroscopy was carried out to study the evolution of chemical bonding of LPCVD SiOxNy films. In order to improve the qualitative analysis of their SiN and Si-O vibrational modes, a correlation between Fourier transform infrared and spectroscopic ellipsometry measurements was established. A shift of infrared peaks position with the increase of relative gas flow ratio is observed. Furthermore, the calculated areas of absorption bands were used to estimate the SiOxNy stoichiometry. This quantitative analysis was proved with an adequate method in the literature. We found a decrease of x values from 1.94 to 1.26 and an increase of y from 0.04 to 0.49, when the NH3/N2O gas flow ratio increases. This behavior was explained by the increase of nitrogen content as well as the decrease of oxygen content in the SiOxNy film.