The combined use of an ultrahigh vacuum tribometer and a variety of surface science techniques has enabled us to explore the tribological properties of interfaces between Ni(100) surfaces and to observe phenomena attributable to molecular layering. Friction measurements have been made between a pair of clean Ni(100) surfaces, modified by the presence of adsorbed atomic sulfur with and without adsorbed ethanol. Friction measurements made with ethanol coverages ranging from 0 to 10 monolayers (ML) on each Ni(100) surface reveal that the friction coefficient is discontinuous in coverage and can be correlated to the coverage dependence of the ethanol desorption energy. During shearing, sliding never commences between clean Ni(100) surfaces or sulfided Ni(100) surfaces without adsorbed ethanol. In the submonolayer coverage regime of either atomic sulfur or adsorbed ethanol, the behavior is characterized by a high friction coefficient (mu (s) > 5.5) accompanied by high adhesive forces (mu (ad) = 1.5 +/- 0.7) An abrupt decrease in both the friction coefficient and adhesion coefficient occurs at a coverage of 1 ML of ethanol on each surface. The friction coefficient drops to mu (s), = 3.1 +/- 1, while the adhesion coefficient is lowered to mu (ad) approximate to 0.25. At coverages between 1.0 and 2.5 ML of ethanol on each Ni(100) surface, the static friction coefficient decreases in a stepwise manner that is correlated with discontinuities in the ethanol desorption energy. This stepwise decrease in both the friction coefficient and the desorption energy may be due to molecular layering of the ethanol.