The rf-sputtering rate of SiO2 decreases rapidly when comparatively small partial pressures (∼10−5 Torr) of oxygen are added to the argon-sputtering gas. The magnitude of the decrease is also dependent on the rf-input power. Both of these phenomena are explained by a model which assumes that the sputtering rate is lowered by the replacement (from the gas) of oxygen atoms which have previously been sputtered out of the SiO2 target.