The surface chemistry of silicon exposed to reactive XeF2 gas and the chemisorption of SiF4 on Si at -150 and 25 °C have been studied using XPS and AES. While SiF4 can be condensed at -150 °C, XeF2 is dissociatively chemisorbed and Xe does not stick on the surface. For both Si/SiF4 and Si/XeF2 at 25 °C, a layer of SiF2-like surface species is identified from the characteristic core level chemical shifts. The formation of this fluorinated surface layer hinders the adsorption of SiF4, but XeF2 reacts with this layer to form volatile SiF4. The behavior of fluorine chemisorption on silicon is illustrated for the first time and the role of surface fluorine in the silicon etching process is discussed in light of the new results.