Acetylation of 2-methoxynaphthalene (2MN) with acetic anhydride (AA) was carried our over a HBEA 15 zeolite (framework Si/Al ratio of 15) under the following conditions: batch reactor, 500 mg of zeolite, 35 mmol of 2MN and 7 mmol of AA, 4 cm(3) of solvent, temperature of 90 degrees C, 120 degrees C or 170 degrees C (generally 120 degrees C). In addition to acetic acid, the main reaction products are 1-acetyl-2-methoxynaphthalene (I) and 2-acetyl-6-methoxynaphtalene (LI); 1-acetyl-7methoxynaphthalene (III) is formed in low amounts and 2-acetyl-3-methoxynaphthalene (IV) in trace amounts. I, which initially is preferentially formed, undergoes isomerization into II and III and also deacylation afterwards, However, high yields into isomer II, which is a precursor of the anti-inflammatory Naproxen, can be obtained by operating at relatively high temperatures (greater than or equal to 170 degrees C) in the presence of a solvent of intermediate polarity such as nitrobenzene. The solvent polarity has a significant effect on the reaction rates and on the selectivity to acetylation, isomerization and deacylation. Very polar solvents such as sulfolane, which compete with the reactant molecules for diffusion inside the zeolite micropores and for adsorption on the acid sites, reduce significantly the reaction rates. Low acetylation and isomerization rates and high deacetylation rates are found with non-polar solvents, such as 1-methylnaphthalene which cannot solvate the acylium ion intermediates. Adsorption experiments suggests that all the acetylmethoxynaphthalene products are mainly formed inside the zeolite micropores.