Adsorption of 11H-eicosafluoroundecanoic acid from a hexane solution onto fluorine-terminated Si(111) surface has been examined using X-ray photoelectron spectroscopy. The decrease in the F1s peak intensity assigned to the surface Si-F bond is accompanied by a quantitative increase in the F1s and C1s peak intensities ascribed to the fluorocarbon chain of the acid. It is shown that each molecule is covalently bonded to the silicon surface giving monomolecular coverage.