Measurements have been made of the coefficient of friction of a steel pin sliding on a steel ring lubricated with three commercial greases, various experimental greases or pastes, and other components of greases. After break-in at room temperature, heat was applied which reduced the friction of the greases to about half their room temperature value. The friction increased on cooling. The opposite effect was observed with base oil. The phenomenon was demonstrated in a ball thrust bearing operating under boundary conditions as a reduction in torque with heating and an increase in torque on cooling. The reduction in friction with heating was found to be due to the soap thickener. Pastes of CaCO3, fumed silica, and clay in oil, the base oil, and octadecane did not cause a reduction in friction. It was concluded that grease thickeners influence the friction of steel sliding on steel because of the formation of films on the surface of the metal.Presented as an American Society of Lubrication Engineers paper at the Lubrication Conference held in Rochester, New York, October 15–17, 1963.