A model describing the relationship between friction coefficient and surface adsorption of a two-component lubricant (i.e. base lubricant and one additive) is developed for boundary lubrication. This model relates the coefficient of friction to the extent of metal-metal contact or, inversely, the fractional surface coverage of the lubricant. Using a suitable adsorption isotherm, an expression for friction coefficient is obtained, relating the friction coefficient to the additive concentration and the adsorption energy. The proposed model and the selected adsorption isotherm are experimentally verified by testing several model additives in a petroleum-base lubricant. An excellent correlation is obtained between the adsorption-free energies calculated from the friction coefficient data and the adsorption-free energies determined from adsorption studies. The ranking order of additives based on friction coefficient is explained in terms of the molecular structure and the structural effects on the adsorption-free energy.Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985