A series of asymmetrical tetraalkylphosphonium ionic liquids were synthesized and evaluated as a new kind of lubricant for the contacts of steel/A1 using an Optimol SRV oscillating friction and wear tester in ambient condition. The phosphonium ionic liquid shows excellent tribological performance and is superior to the conventional ionic liquids 1-ethy-3-hexylimidazolium hexafluorophosphate (P206) in terms of anti-wear performance and load-carrying capacity. The chemical compositions of the boundary film generated on different contact surfaces were analyzed on a scanning electron microscope with a Kevex energy dispersive X-ray analyzer attachment (SEM/EDS) and X-ray photoelectron spectrometer (XPS). The friction-reduction and anti-wear mechanism were proposed to originate from the layered structure of ionic liquid under boundary lubrication and the tribochemical reaction of anions with the fresh surface, respectively. (c) 2006 Elsevier B.V. All rights reserved.