The low coefficient of friction for Teflon sliding on Teflon, as observed previously by others, has been found in this study to hold only if low sliding speeds and newly prepared surfaces are used. At high sliding speed, the nature of the surface is irreversibly changed such that subsequent sliding at low speed reveals a two‐ to threefold increase in the friction coefficient.In addition to the effect of high speed sliding, there are reversible changes in the friction of Teflon on Teflon dependent in part on temperature. As the temperature of Teflon is increased from below room temperature, a sharp and pronounced increase in the coefficient of friction is observed in the vicinity of 20°C. The existence of a phase transition in Teflon at this temperature indicates a close correlation between the frictional and the structural properties of this material.