Timken OK load tests along with friction and wear measurements for a series of gear oils containing sulfurized olefin (SO) and oleyl amine salt of acid 2-ethylhexylphosphate (AP) have been carried out. Electron probe microanalyses have been made on the worn surfaces of Timken test blocks lubricated with the gear-oils. The main results are as follows: (1) A phosphorus-containing sulfur-rich EP-film is formed by metal-to-metal contact in the initial stage of the Timken test where the wear scar width increased, rapidly and friction fluctuated. (2) Later, a sulfur-containing phosphorus-rich EP-film is gradually formed and the wear scar width and friction become stabilized. The formation of the phosphorus-rich EP-film results in stabilizing the wear scar width and friction. (3) In the later stage of the test, phosphorus seems to form some stoichiometric compounds with oxygen in the EP-film. (4) Synergistic effects in load carrying and wear reducing abilities are seen in both SO and AP. The adsorption of the ad...