Reactive ion etching of Si and Ge in SF6–O2is investigated. Etch rate shows that Si etching is selective with respect to Ge in SF6–O2(O2<50%); the reverse is observed in SF6–O2(O2≳50%). In agreement with the compared evolution of the F and O concentration in the plasma and of the etch product formation rate obtained respectively, by optical emission spectroscopy and mass spectrometry, the x‐ray photon spectroscopy surface analysis reveals that the growth of a SiOxFylayer quenches the Si etching whereas the formation of GeOxFydoes not inhibit Ge etching. Using a simple model, an experimental surface reactivity is defined and expressed in function of the experimental data. Results suggest a different behavior for sulfur and oxygen on both materials.