Provided is an ionic liquid, represented by the formula K.sup.+A.sup.-, having a very low content of impurities, especially a low content of halogen ions and/or a low content of alkali metal, wherein K.sup.+is a quaternary ammonium ion, for example an imidazolium ion. Also provided is a quaternarization process for making the ionic liquid in reacted with an acid ester such as a sulfate or phosphate.