Acylation of 2-methoxynaphthalene with acetic anhydride has been carried out with a tridirectional 12-member ring pore zeolite named ITQ-7, which has a slightly smaller pore diameter than zeolite Beta. ITQ-7 is as active as Beta but gives better selectivity to 2-acetyl-6-methoxynaphthalene (2-AMN). This is due to the differences in the relative diffusion coefficients of 2-AMN and 1-acetyl-2-methoxynaphthalene (1-AMN), whose ratios are 2.7 and 15.5 in Beta and ITQ-7, respectively. In general, it can be said that when decreasing the zeolite crystallite size, the reaction rate increases, although the selectivity to 2-AMN decreases. (C) 2000 Academic Press.