The techniques of laser and electron ionization mass spectrometry have been employed to study the thermal etching of Si(110) by F2 and NF3 at substrate temperatures between 300 and 1200 K. By two-photon resonance-enhanced ionization of SiF2 via the B̃1B2 state, the apparent activation energy for gaseous silicon difluoride production was found to be 8.9 ± 0.3 and 22.1 ± 1.7 kcal/mol for F2 and NF3, respectively. SiF was not detected. An extensive search for SiF3 during etching by F2 at 1000 K, by means of resonance ionization from 320 to 325 and from 416 to 510 nm, also showed no signs of the species. Both SiF and SiF3 are thermochemically unimportant etch products under the conditions employed. In F2 etching, SiF4 and total silicon fluoride ΣSiFx + signals as measured by electron ionization rose rapidly at lower temperatures and stabilized between 700 and 900 K before rising again. No such behavior was observed for SiF2 production from F2 or for the products formed in NF3 etching. Apparent activation energies for total silicon fluoride and SiF4 production are similar. For F2, they were found to be about 9 kcal/mol in the low-temperature region, and for NF3 both were measured to be about 21 kcal/mol. A proposed reaction mechanism explaining these and related results is discussed.