The antiwear performance of tricresyl phosphate dissolved in 11 kinds of base oils of different polarities has been studied. A mineral oil (MO) and an alkylnaphthalene (AN) were used as non-or low-polarity base oils. Five polyol esters (POEs), i.e., three trimethylolpropane esters and two pentaerythritol esters, and four polyalkylene glycols (PAGs) were used as the high polarity base oils. A four-ball friction apparatus was used to determine the anti-wear performance, using commercially available bearing steel balls as test specimens. All the TCP-formulated base oils showed optimum concentration characteristics for minimizing wear. It is shown that the order of the optimum concentration for POEs and PAGs can be reasonably explained by the interaction between additive and base oils, by using their solubility parameters and molecular sizes. These oils' dielectric constants showed little correlation with optimum concentrations. The dielectric constant showed large effects on such non- or low-polarity oils as MO and AN. A good correlation of the optimum concentration for all the base oils was obtained when it was arranged as a function of (SPbase/SPTCP)14.2(MWbase/MWTCP)−2.78(ε)18.6.Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999