The tribological properties of ionic liquids were outlined from the viewpoint of tribo-chemistry. The tribological properties of two hydrophobic anions, trifluorotris (pentafluoroethyl) phosphate (FAP) and bis (trifluoromethylsulfonyl) imide (TFSI), were compared by a pendulum type friction test. FAP salts of imidazolium possess much better lubricity than TFSI salts of imidazolium. Tribo-chemical reactions of the ionic liquids were studied by time-of-flight secondary ion mass spectroscopy. It was found that content of phosphorus on the inside wear track was higher than that on the outside wear track. The results indicate that reaction of FAP took place under the tribo-chemical conditions. Straight-chain carboxylic acids were found to improve tribological properties of the ionic liquids. Direct interaction of the acid with the rubbed surfaces was confirmed by using stable isotopic tracers.