To understand the chemical behavior of EP additives in sliding contacts, the concept of an anionic mechanism of the elementary sulfur and organo-sulfur compounds has been proposed. The model is based upon the ionization of these compounds by the action of exoelectrons. It has been shown that disulfides exhibit more efficient EP performance than monosulfides because lower energy is needed for the formation of the same number of RSΘions from disulfides. The corrosive action of the elementary sulfur in the friction process can be explained by the formation of ions from SΘto S8Θ. An attempt to interpret the difference in EP performance of diphenyl sulfide and dibenzyl sulfide has also been made as well.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Hartford, Connecticut, October 18–20, 1983