The fluorinated surface layer of poly(4-methyl-1-pentene) membranes exposed to a dilute stream of fluorine gas has been characterized with X-ray photoelectron spectroscopy. The concentration and profile of reacted fluorine as a function of exposure time is determined. A computer routine was employed to deconvolute the poorly resolved carbon spectra after various fluorine exposure times. The concentrations of mono-, di-, and trifluorocarbon groups thus determined were used to propose specific structures of PMP at the surface after 1 and 15 min of fluorination. The carbon spectra collected at electron takeoff angles of 15°, 30°, and 90° were also deconvoluted, giving insight into the placement of fluorine as a function of depth. Oxygen is incorporated into the polymer during the fluorination reaction, and the O1s spectra was deconvoluted to determine how the oxygen is bound.