Oligomerization of isobutene has been investigated using FeCl3-modified HY zeolite catalysts in order to produce triisobutenes or remove/separate isobutene from C-4 streams. Stable isobutene conversion and high selectivity for trimers are attained over the modified zeolite with high concentration of Lewis acid sites, suggesting that a potential trimerization catalyst can be obtained easily by loading a Lewis acid into an acidic zeolite catalyst.