The contact area of the plate of a ball/plate (both consisting of hardened M-50 steel) sliding elastohydrodynamic contact run on trimethylolpropane triheptanoate with or without tricresylphosphate (TCP) additive was optically profiled periodically with a phase-locked interference microscope (PLIM), both before and after exposure to alcoholic hydrochloric acid. As scuffing was approached, the change of profile within the contact region changed much more rapidly by the acid probe and assumed a constant high value after scuffing. A new nonetching metallurgical phase was found in the scuff mark, which was apparently responsible for the high reactivity. Soaking the metal surfaces in the lubricant before running affected the subsequent reactivity.