Three different types of silicon oxynitride layers were fabricated on thermally oxidized silicon wafers by low-pressure chemical vapor deposition using different gas flow ratios NH₃/N₂O with a fixed gas flow of SiCl₂H₂. The differences of the physical properties of the resulting layers were investigated by spectral ellipsometry. Electrical and electrochemical characterization were performed using high frequency capacitance-voltage measurements on metallized samples. The silicon oxynitride layers showed sensitivities towards hydrogen and alkali ions that are between the sensitivities of silicon dioxide and silicon nitride while having stability comparable to silicon nitride.