An ultrahigh vacuum multiple-beam setup has been designed to study surface reactions that are of importance in plasma etch processes. The setup consists mainly of five beams (reactive neutrals, ions, electrons, CFx radicals, and photons) all of which can be focused on the same sample area, and a quadrupole mass spectrometer detecting only molecules desorbing from this area. Data are reported on spontaneous etching for the Si(100)/XeF2 system. Both the incident flux of XeF2 on the sample and the desorbing fluxes of SiFx products and nonused XeF2 were measured quantitatively. The reaction of XeF2, the production of SiFx species and the accumulation of fluorine on the silicon surface were studied as a function of temperature (300–900 K) for XeF2 fluxes of 0.24 and 1.04 monolayers/s. For the reaction probability of XeF2 an exponential increase with temperature from 11% at 300 K up to 50% at 900 K was found. The main product was found to be SiF4 at 300 K gradually becoming replaced by SiF2 at temperatures higher than 600 K. A remarkable result is the accumulation of a rather thick SiFx reaction layer on the etched surface. From thermal desorption spectra the fluorine content was calculated to be 38 monolayers at 300 K decreasing rapidly for higher temperatures. This suggests that fluorine diffusion into the silicon bulk might be of importance in the etching process.