The surface chemical interactions between boundary or extreme pressure (EP) lubricant additives and the mechanical components they protect Often determine the performance and life of marginally lubricated and highly loaded mechanisms. Most of these additives have been developed for use with steel components, and a lack of fundamental understanding regarding the surface bonding and reactivity occurring in the mechanical contact severely limits the application of these additives to mechanisms incorporating new materials. This work examines the chemical interactions between the EP additive lead naphthenate (Pbnp) and a series of titanium-based materials (Ti, TiB2, TiC, TiN, and TiO2). These X-ray photoelectron spectroscopy (XPS) studies show that Pbnp applied via solution chemisorbs to all df these materials and that the surface activity is enhanced by damaging the substrate (i.e., wear) during exposure. In addition, both Ti and TiB2 are capable of chemically reducing Pbnp to metallic lead, while TiO2 stabilizes and oxidized form of lead. Neither TiC nor TiN, the most widely used of these materials for antiwear coatings, is capable of chemically reducing Pbnp or of retaining significant amounts of Pbnp fragments at elevated temperature. The surface chemical reactivity of these materials is related to their relative ionicities and surface electronic structures. The results indicate that Pbnp is probably not a good candidate as an EP additive for TiC and TiN coatings.