Adsorption of fluorinated molecules from a liquid solution onto a fluorine-terminated Si(111) surface has been examined using X-ray photoelectron spectroscopy (XPS). The observed interchange of an F 1s peak from the terminal Si-F at the surface by the core electron peaks ascribed to the adsorbed molecule indicates the formation of an adsorbed monomolecular layer on the surface. In the case of molecules with a linear fluoroalkyl chain, the adsorbate core peaks are observed to shift to lower binding energies with increasing coverage of the Si surface by chemisorbed molecules: the maximum energy shifts, observed for 3-perfluorohexyl-1,2-epoxypropane, are -0.5(6) eV for C 1s and -0.3(5) eV for F 1s. This decrease in binding energy can be explained by the increasing relaxation shift caused by the increase of molecular density which permits a larger contribution of the van der Waals attraction among adsorbed molecules. This could be the first observation of a core-level shift due to lateral interchain interactions among ad-molecules, which shows that XPS is also useful for examining intermolecular interactions in the adsorbed layer.