The spontaneous thermal etching of silicon by CF3free radicals has been studied in a very‐low‐pressure photolysis reactor. The radical is produced by infrared multiphoton dissociation of either hexafluoracetone or CF3I, and is allowed to react with a temperature‐controlled silicon sample (560–745 K). Mass spectrometry is used to measure the extent of dissociation of the precursor gas and the formation of product molecules, C2F6and SiF4. The etch rate of the silicon is determined from the SiF4production. Resonance‐enhanced multiphoton ionization of CF3is used to determine the density and time history of the radical in the reactor. The measurements of the etch rate and CF3density are combined to derive the reaction probability. CF3etches silicon much more slowly than F atoms and at a rate comparable to molecular F2. A carbon layer, that is deposited on the silicon by the radicals, inhibits, but does not stop, further etching. Experiments on the etching of silicon by F2were performed both to validate the reactor design and to prepare the silicon surface for the CF3studies.