Liquid phase acylation of naphthalene with acetic anhydride was investigated over large pore zeolites faujasite, mordenite and beta, It has been shown that mordenite and faujasite were easily deactivated in contrast to zeolite beta, The highest activity and long term stability of zeolite beta was achieved with Si/Al ratio about 37. At lower Si/Al ratios a rapid deactivation of zeolite beta was observed, while at Si/Al ratios higher than 60 the activity of this zeolite was very low. Zeolite beta was found to be an optimum catalyst for naphthalene acylation exhibiting a selectivity to 2-acetylnaphthalene over 80%. Deactivation of the zeolite is probably caused by the decomposition of the acylating agent followed by condensation reactions of its decomposition products and/or formation of higher aromatic compounds which are not easily desorbed from the zeolite channels. The excess of naphthalene in the reaction mixture and stepwise adding of acetic anhydride improved the naphthalene conversion and the stability of the zeolite used.