The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(lll) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.