Experience has shown that the lubrication behavior of steel bearings can be significantly improved by a pretreatment called oil aging. Neat tricresyl phosphate (TCP) is normally used for this process (1)—(4). In this study, however, two hydrocarbon-type oils, each containing one percent TCP, were used for oil aging 52100 steel specimens under a variety of experimental conditions. Each coating formed as a result of the oil-aging process was analyzed by electron spectroscopy for chemical analysis (ESCA). The results indicate that microscale oxidation of the base oil and preferential adsorption phenomena within the oil system components are necessary for the formation of a desirable coating, including a phosphate type of material. The absence of aromatics and polar contaminants in the lubricant is also a determinant factor for the formation of a phosphate coating. The nature of this phosphate and the mechanism of its formation are explained.Presented at the 39th Annual Meeting in Chicago, Illinois, May 7–10, 1984