Abstract: A non-aqueous lubricating oil composition comprises a major amount of a base oil and a minor amount of an additive which is present in solution in the base oil and is a non-halide, non-aromatic ionic liquid which is a salt of general formula C+A?, with the cation, C+, being a quaternary phosphonium or quaternary ammonium ion having four hydrocarbyl groups, optionally containing heteroatoms, which hydrocarbyl groups do not all have the same number of carbon atoms, there being at least one long chain hydrocarbyl group having greater than 4 carbon atoms and at least one short chain hydrocarbyl group having fewer carbon atoms than each of the long chain hydrocarbyl groups, and the anion, A?, comprising at least one oxygen atom and having an ionic head group attached to at least one alkyl or alicyclic hydrocarbyl group which has at least four carbon atoms and optionally at least one heteroatom. The ionic liquid may be used as an anti-wear component in a lubricating oil composition.