The reliability of oxynitrides with thicknesses from 4.0 nm down to 2.6 nm. was investigated. The oxynitrides were grown by rapid thermal processing at reduced pressure in gas atmospheres of N2O, a 50% mixture of N2O and O-2, and O-2. The concentrations of the nitrogen incorporated into the oxide during growth were found to be dependent on both, oxidation pressure and oxidation atmosphere. Reliability tests were performed by constant voltage stress on MOS capacitors. The measurements revealed that oxynitridation in N2O containing ambient can improve reliability even for oxides thinner than 3 nm. The nitrogen concentration, which is beneficial to oxide reliability, shows strong dependence on oxide thickness.