Ionic liquids are often contaminated by trace-colored impurities. In this paper, a solid-phase extraction (SPE) method for purification of undiluted imidazolium ionic liquids was described. A SPE apparatus with octadecylsilyl (ODS) solid phase was proposed to separate these impurities from ionic liquids via hydrophobic interaction. Solid-phase extraction with ODS can improve the spectroscopic quality obviously for the tested ionic liquids. Without introducing any solvent to ionic liquid, the purified ionic liquid can be used directly in spectroscopic measurements. It was found that short alkyl chain ionic liquids were purified more efficiently than the long alkyl chain ones. This method can be used to purify the hydrogen-bonding anion containing ionic liquids, which cannot be decolorized efficiently by conventional active charcoal method. In comparison with other existing purification methods, ODS is advantageous to solve the purification problem of ionic liquids. (c) 2007 Elsevier B.V. All rights reserved.