A study was performed to establish the transition temperature of two additives in cetane under severe conditions of pressure × velocity, which gave a surface temperature rise within the contact of up to 150°C. The interface temperature was measured using the slider as one leg of a thermocouple and the disk as the other; hence, an accurate interface temperature could be determined as the velocity and load varied. The test specimens were M50 tool steel, AISI 52100 steel and type 440C stainless steel balls against a fully hardened tool steel disc. The results complemented previous work which showed the identical trends but at slightly different absolute temperature levels.