In silicon etching in SF6/O2 plasmas, an oxidation threshold appears when the oxygen content is large enough. A SiOxFy passivation layer is formed under such conditions. This threshold is reached at lower oxygen proportions if the substrate is cooled down to cryogenic temperatures. In this article, we present a mass spectrometry study of this oxidation threshold in different experimental conditions (temperature, source rf power, self-bias) on bare silicon wafers. The presence of the threshold is clearly evident in the signals of many ions, for example, SiF3+, F+, and SOF2+. This helps us to determine the main reactions which can occur in the SF6/O2 plasma in our experimental conditions. This threshold appears for higher oxygen proportions when either the source power or the chuck self-bias is increased. The ion bombardment transfers energy to the surface and makes the film desorb. A model, describing the oxygen coverage as a function of the parameters mentioned above, is proposed to interpret these results. Data presented in this article give another point of view of the cryogenic etching process. They contribute to explain how anisotropic profiles can be achieved at low temperature. Surfaces subjected to ion bombardment (the bottom of the structures) are below the oxidation threshold while the structures sidewalls, not subjected to ion bombardment, are in passivating regime.