The alkylation of naphthalene using tert-butanol in cyclohexane over a dealuminated H-Mordenite (HM) zeolite has been optimised to give a 60% yield of 2,6-di-tert-butylnaphthalene with a 2,6/2,7 ratio of over 50. This has been achieved by varying the reaction time, temperature, pressure, solvent, amount of tert-butanol, solvent and catalyst, Si/Al ratio of the catalyst, and the mode of addition.