Silicon oxynitride thin films deposited by plasma enhanced chemical vapor deposition on various substrates have been characterized by infrared reflection absorption spectroscopy. Shifts of the LO Si-O vibrational band to lower wave numbers have been observed with increasing N content and also with the thickness of the films. We observe that the ratio of LO Si-O vibrational mode wave number to him thickness (v/t) varies linearly with the ratio of refractive index to thickness (n/t). This linear relationship does not depend on the nature of substrates on which films are deposited (Al/Si, Si, Si/Al). Our results show that the composition of silicon oxynitride films could be determined directly by measuring the LO Si-O mode position.