Reaction of sodium atoms with poly(tetrafluoroethylene) (PTFE) under ultrahigh vacuum conditions has been followed by X-ray photoelectron spectroscopy (XPS). Surface defluorination and formation of NaF are found to occur. Initial stages of reaction result in desorption of NaF molecules; this is followed by permeation of Na adatoms into the polymer subsurface region to yield trapped NaF moieties.