2-Methoxynaphthalene (2-MN) has been acylated with acetic anhydride on polymorph C of Beta (ITQ-17), and 2-acetylmethoxynaphthalene (2-AMN) and 1-acetylmethoxynaphthalene (1-AMN) have been obtained. The diffusivity of the two isomer products, as determined by molecular dynamics, shows that the ratio of the diffusion coefficients for 2-AMN/1-AMN is one order of magnitude larger for polymorph C than for polymorphs A and B (Beta zeolite). This corresponds very well with experimental acylation results that show a higher selectivity for 2-AMN on polymorph C when reaction is carried out in batch as well as in a fixed-bed continuous reactor. (C) 2003 Elsevier Science (USA), All rights reserved.