Dielectric properties of various silicon oxynitride thin films (similar to 30 nm), deposited by low pressure chemical vapour deposition (LPCVD) technique, are studied by capacitance-voltage (C-V), current-voltage (I-V) and ageing under constant current injection (V-t). It is found that the nitrogen-rich oxynitride layers have lower mid-gap interface trap sheet densities, lower fixed charge sheet densities and higher breakdown fields in comparison to oxygen-rich oxynitride layers. The experimental data for the dielectric constant are compared to the Bruggeman theory applied to the mix of silicon dioxide SiO2 and silicon nitride Si3N4 A good agreement is obtained except for the oxygen-rich silicon oxynitride films where the SiOxNy porous structure and leakage current are responsible for errors. A Poole-Frenkel thermal-emission is shown to be the dominant conduction mechanism in the silicon oxynitride films while the leakage-current increases with increasing O/Si ratio.