The Friedel–Crafts acylation of anisole (AN) with acetic anhydride (AA) in liquid phase catalysed by bulk and silica-supported heteropoly acids (HPA), mainly H3PW12O40 (PW), has been studied. The PW exhibit very high activity, yielding up to 98% para and 2–4% ortho isomer of methoxyacetophenone (MOAP) at 90–110°C and an AN/AA molar ratio of 10–20. Catalyst pretreatment is essential; the activity passes a maximum at a pretreatment temperature of 150°C. The acylation of anisole appears to be heterogeneously catalysed; no contribution of homogeneous catalysis by HPA was observed. PW is almost a factor of 100 more active than the zeolite H-beta, which is in agreement with the higher acid strength of HPA. The PW catalyst is reusable, although gradual decline of activity was observed due to the coking of the catalyst. The acylation is inhibited by product because of adsorption of MOAP on the catalyst surface. The ratio of adsorption coefficients of MOAP and anisole has been found to be 37 at 90°C. Anisole acylation is first order in acetic anhydride, the order in catalyst is 0.66, and the apparent activation energy is 41 kJ/mol in the temperature range of 70–110°C. In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-beta. Evidence is provided that the activity of HPA in toluene acylation is inhibited by preferential adsorption of acetic anhydride on the catalyst.