A new purification method of ionic liquids made from trialkyloxonium salts (Meerwein reagents) has been established. H-1 and F-19 NMR spectroscopy clarified the presence of 1-methylimidazolium ([Hmim](+)) and trifluoro(1-methylimidazole)boron ([BF3(mim)]) as impurities occurring in the crude ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim]BF4). The Lewis acidic parts of these impurities, that is, H+ and BF3, were demonstrated to be able to transfer to dimethylamino groups of the weakly basic anion exchange resin (DOWEX MARATHON WBA) in aqueous solutions, releasing 1-methylimidazole (mim). The adsorption isotherms of [Hmim]BF4 and [BF3(mim)] resulted in the equilibrium constants (K) of the reactions with the resin ([Hmim]BF4: log K approximate to 1.7, [BF3(mim)]: log K = -0.17 +/- 0.03), indicating that the impurities show a difference in the reactivity with the resin. The volatiles (mim and water) are possible to be removed by evaporation under reduced pressure. Although intense reddish brown color was given to the residual oil of [emim]BF4, further purification and decolorization have been achieved by using silica-gel column chromatography (eluent: acetone) and treatment with activated carbon in water. Finally, the purified [emim]BF4 was successfully obtained as an almost colorless oil. The resin after use can be regenerated simply by washing with 1 M NaOH aq and deionized water.