Antiwear additives are necessary components of motor oils in order to maintain satisfactory wear rates of automobile engines, particularly in the valve train area. Even though much has been accomplished to minimize wear, the processes by which antiwear additives function to mitigate wear are not completely known. A mechanism is proposed which includes antiwear additive—surface interaction by either adsorption followed by reaction or direct reaction. This mechanism is shown to be consistent with previous experimental data. Evidence is shown indicating that surface sulfide is not the sole species responsible for boundary lubrication with organosulfur compounds. The formation of sulfide rather than the thermodynamically preferred sulfate is indicated to be the result of kinetic limitations which restrict oxygen and produce a “reducing atmosphere” during reaction on the surface. A mechanism for producing wear debris is proposed. This mechanism relies on diffusion and segregation of antiwear additive elements into grain boundaries followed by subsequent fracture. This model is shown to be consistent with experimental observations.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in New Orleans, Louisiana, October 5–7, 1981