Alkylation of isobutane with 2-butene has been conducted in ionic liquid media, 1-alkyl-3-methylimidazolium halides-aluminum chloride encompassing various alkyl-groups (butyl-, hexyl- and octyl-) and halides (Cl, Br and I) on its cation and anion, respectively. The emphasis has been to delineate the role of both cations and anions in this reaction. The ionic liquid bearing a large alkyl group on the cation, [C8mim], displayed relatively higher activity than small ones, [C6mim] or [C4mim], with the same anionic composition. This was due to the high solubility of reactants in the former. Among the ionic liquids with different halide groups, bromides such as [C8mim]Br-AlCl3 showed outstanding activities because of the high inherent acidity relative to others. From the 27Al-NMR study, a major peak at ~99.5 ppm corresponding to [AlCl3Br]- was observed. Moreover, the largest peak at 1570 cm-1 in FT-IR analysis characterized the anion possessing strong acidity. The catalytic activity and the amount of the target product, tri-methylpentane (TMP), increased with the concentration of anion. This is mainly attributed to high concentration of strongly acidic [Al2Cl6Br]- which can react with hydrogen atom at the 2-position of the imidazolium ion to form Brønsted acid. However, the ionic liquid with strong acidity (X=0.58) was rapidly deactivated due to its high sensitivity to moisture which causes the decomposition of ionic liquids. Since the acidity of anion increased with temperature, the imidazolium ion readily decomposed at high temperature and its solubility and Brønsted acid sites accordingly reduced. The optimal reaction temperature for high catalytic activity and TMP selectivity was observed at 80 oC. Catalytic behavior of an ionic liquid, [C8mim]Br-AlCl3, was compared with that of sulfuric acid, a standard commercial catalyst. Under optimum experimental condition, the ionic liquid showed higher activity and lower TMP selectivity than sulfuric acid due to its higher acidic nature and less amount of Brønsted acid sites, respectively.