A series of very simple radioactive chemical tests has been carried out to measure reaction of dibenzyl disulfide on steel. By comparing the results with friction experiments under the same conditions, some of the requirements of extreme pressure lubrication have been determined. Current views on the mechanism of reactions of dibenzyl disulfide have been confirmed and it has been demonstrated that only a thin EP film is needed to give boundary lubrication. In particular, an important additive interference effect has been noted between extreme pressure activity and high temperature detergents.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Atlanta, Georgia, October 16–18, 1973