Thrust ball bearing studies in which the surface topography of bearing races was examined with an electron microscope and in which additive elements were measured on ball surfaces by X-ray fluorescence have indicated that surface films of appreciable thickness are formed by oil additives and that the additive composition controls both surface topography and surface coating composition. Further, additive concentration, friction-time, and oil temperature studies have shown that the composition and/or thickness of the surface film is a function of the additive concentration, and suggest that most of the surface-active additives read in some manner rather than physically adsorb on surfaces. However, friction measurements correlate only to a limited degree with either the measured surface film composition or the observed topography.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Rochester, New York, October 15–17, 1963.