The behaviours of two heteropoly acid (HPA) catalysts, one supported on a commercial silica and the other on a silica-zirconia mixed oxide, were compared in the acylation of anisole with acetic anhydride. Both supported catalysts and the latter support, but not the silica, were active for the reaction, although all underwent a deactivation process before achieving complete conversion of anisole to methoxyacetophenone. The greatest yield to p-methoxyacetophenone was obtained for HPA/SiO2 while the extent of deactivation was in the order HPA/SiO2-ZrO2 > SiO2-ZrO2 > HPA/SiO2. Although acetate species accumulated on the SiO2-ZrO2 and HPA/SiO2-ZrO2 catalysts during the reaction, these species are not thought to play a role in the deactivation process. The relative extent of deactivation was consistent with the calculated ratios of adsorption coefficients which determine the strength of adsorption of product to that of anisole and indicate that product inhibition is greatest in the case of HPA/SiO2-ZrO2. Calculated absorption coefficients for pyridine adsorbed on the samples failed to distinguish between the nature of the HPA species dispersed on the two different supports. (C) 2004 Elsevier Inc. All rights reserved.