An argon plasma with a small amount of oxygen in a radio-frequency magnetron sputtering system has been studied experimentally. A Langmuir probe has been used to measure the ion density, electron temperature and plasma potential in the plasma, whereas an energy-resolved mass spectrometer has been used to investigate the ions flowing towards the growing film. It has been found that the argon ion density decreases when the oxygen gas is added to the plasma above a certain flow. A discussion on which process is responsible for this effect is presented. (c) 2005 Elsevier B.V. All rights reserved.