HCl-modified triethylamine aluminum chloride ionic liquid (Et₃NHCl–2.0AlCl₃–HCl) was prepared and ²⁷Al NMR measurements were carried out. The acidity of the ionic liquids (ILs) was characterized by FT-IR with using pyridine as a probe molecule and the Hammett acidity function was determined by UV-vis spectroscopy. The catalytic performance of the ionic liquids for the synthesis of 2,6-dimethylnaphthalene (2,6-DMN) with high selectivity by the transalkylation of 2-methylnaphthalene (2-MN) with 1,2,4,5-tetramethylbenzene (TeMB) was investigated. Under the optimal reaction conditions, 15.6% conversion of 2-MN and 100% selectivity of 2,6-DMN in the dimethylnaphthalenes were obtained.