Previous work has shown that reaction of zinc di-n-octyldithiophosphate with peroxy radicals in laboratory experiments led to products which were ineffective as antiwear agents. These products have now been identified and a material balance obtained by the application of several analytical techniques. The principal reaction product was the disulfide species tetra-n-octylthioperoxydiphosphate. This compound was synthesized, tested, and found to be less effective than zinc di-n-octyldithiophosphate as an antiwear agent.Presented at the 38th Annual Meeting in Houston, Texas, April 24–28, 1983