Diamond (100) substrates have been fluorinated under ultrahigh vacuum conditions with both atomic and molecular fluorine. X‐ray photoelectron spectra of the resulting samples indicate that atomic fluorine, F, reacts efficiently at 300 K producing a saturation coverage of about three quarters of a monolayer (one monolayer &bartil;1.6×1015cm−2) after 40 monolayers exposure. The carbon fluoride adlayer is thermally stable to 700 K but slowly desorbs at temperatures above this. In contrast, molecular fluorine, F2, reacts quite slowly; a saturation coverage of less than one fifth of a monolayer is achieved after several hundred monolayer exposure to F2at temperatures from 300 to 700 K. Diamond surfaces saturated with fluorine atoms showed no loss of fluorine after sequential exposure to beams of H2and O2at temperatures between 300 and 700 K.