Reactive ion etching of silicon and germanium in CF4 -O2 was investigated. Above 20% O2 germanium etching is selective with respect to silicon. In agreement with the evolution of the fluorine and oxygen concentration in the plasma and of the etch products formation rate, surface analysis reveals that the growth of a SiOxFy layer slows down the silicon etching whereas the formation of GeOxFy does not inhibit germanium etching. Using a simple kinetic model, the silicon and germanium reactivity and its dependency with respect to the plasma composition are expressed in function of the experimental data. Results suggest that surface composition controls silicon etching, whereas germanium etching depends only on the fluorine flux on the surface.