The oligomerization of isobutene catalyzed by ionic liquids containing FeCl4- and Fe2Cl7- was conducted in an autoclave. The conversion of isobutene was above 83 wt%, and the selectivity of diisobutene plus triisobutene was above 75% when the mole ratio of FeCl3 to [(C2H5)(3)NH]Cl ranged from 1.2:1 to 2:1. The addition of cuprous chloride to the ionic liquid increased the conversion of isobutene and the selectivity of diisobutene plus triisobutene. The latter reached 90% at the mole ratio of CuCl to [(C2H5)(3)NH]Cl-1.5FeCl(3) of 0.25:1. The reaction pathway of the oligomerization of isobutene catalyzed by iron(Ell) chloride ionic liquids was explained by the carbonium ion mechanism.