Silicon oxynitride thin films were synthesised by the reactive gas pulsing process using an argon, oxygen and nitrogen gas mixture from a semiconductor Si target. Argon and nitrogen were introduced at a constant mass flow rate, whereas oxygen gas was periodically supplied using a rectangular pulsed flow rate. The O-2 injection time T-ON (or duty cycle alpha) was the only varied parameter. The influences of this parameter on the discharge behaviour, on the Si target voltage, and on the resulting chemical composition of the films were investigated. The temporal evolution of the total pressure exhibits exponential shape differing from the rectangular oxygen pulse shape, due to the response time of the gas flowmeter and to the progressive oxidation of the target and the chamber walls. During the T-ON time, the preferential adsorption of the introduced O-2 induces a decay in Si target voltage. Reversion to the nitrided mode is still possible as soon as the O-2 injection is stopped. The elemental analyses assessed by secondary neutral mass spectrometry (SNMS) showed that the O/N ratio within silicon oxynitride films linearly depends on the T-ON time. Increasing the duty cycle alpha over a certain value results in an oxidised steady state formation during the T-ON time. This formation was observed by real time measurements of the emission lines ratio I(O*)/I(Ar*) indicative of the O-2 partial pressure and confirmed by the time derivative of the target voltage. During the T-OFF time, the alternation with the nitrided mode becomes impossible, leading to the specific synthesis of stoichiometric SiO2 films. (C) 2011 Elsevier B.V. All rights reserved.