Silicon oxynitride films were deposited at room temperature using the ECR-PECVD technique. Precursor gases were O-2, N-2 and SiH4. The composition of the films can be controlled by regulating the gases flow ratio. R = (O-2 + N-2)/SiH4 and R' = O-2/SiH4 have proved to be the key deposition parameters. FTIR spectroscopy, AES and ellipsometric measurements were performed in order to characterise the films. A single Si-O/Si-N stretching band is observed in the FTIR spectrum for all compositions, indicating single-phase homogeneous SiOxNy films. FWHM of the stretching band shows a maximum for the composition corresponding to the same number of Si-O and Si-N bonds. Samples cover the whole composition range from silicon nitride to silicon oxide including nitrogen-rich films, even though the gas flow ratio R " = N-2/O-2 during deposition was small (from R " = 1.0 for SiO1.9N0.04 to R " I = 6.7 for SiO0.26N1.2). Silicon oxide composition samples (SiO2.0) show essentially the same TR features as the thermal oxide: Si-O stretching band located at 1072 cm(-1), with a FWHM of 96 cm(-1) and a shoulder/peak ratio of 0.30, while nitrogen-rich samples (SiO0.26N1.2) show a total bonded hydrogen content below 2 x 10(22) cm(-3).