A synergistic effect was observed for systems of phosphites and sulfur-type additives, while no interaction was observed for systems of phosphates and sulfur-type additives. The mechanism of synergism was studied by EPMA, FT-IR, GC-FPD, and MS. From a synergistic mixture, a new compound containing phosphorus and sulfur was detected. The new compound which was separated by distillation showed good antiwear properties comparable with the synergistic system and was identified as a thiophosphate. The synergism between the sulfur-type additive and phosphite may be caused by the formation of thiophosphate. The chemical reaction between them is discussed and a reaction formula is proposed.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Tribology Conference in Atlanta, Georgia, October 8–10, 1985