Isopropylation of naphthalene with propylene was carried out over Ce-, Si-, La-, and Mg-modified mordenite zeolites. The reaction was performed in a 500 mL batch reactor at 300°C, 8 kg/cm2, and naphthalene to catalyst weight ratio of 10:1, with excess propylene. It was attempted to eliminate the external acid sites of zeolites by these modifications. The results indicate that the CVD silanation and magnesium or cerium modification with Mg/Al or Ce/Al atomic ratios less than 0.5 can improve the 2,6-DIPN selectivity of mordenites. The improvement of catalyst performance is ascribed to the selective deactivation of acid sites on the external surface of HM zeolites.