CuCl-based ionic liquid was synthesized by mixing 1-butyl-3-methylimidazolium chloride with purified anhydrous CuCl, and its structures were studied using fast atom bombardment mass spectrometry (FAB-MS). It was determined that Cu(I) anionic species such as CuCl2-, Cu2Cl3-, and Cu3Cl4- existed in the ionic liquid, and these anions were moisture-insensitive and stable in air. CuCl-based ionic liquid exhibited remarkable desulfurization ability in the desulfurization of gasoline when used as an extraction absorbent. The effectiveness of sulfur removal may be attributed to the pi-complexation of Cu(I) with thiophene, which shows promise as an approach for the deep desulfurization of motor fuel.