The Friedel-Crafts benzoylation of anisole with benzoic anhydride to yield 4-methoxybenzophenone has been carried out in a range of ionic liquids using zeolite catalysts. The rates of reaction were found to be significantly higher using ionic liquids compared with organic solvents. Continuous-flow studies of successful ionic liquid systems indicate that the bulk of the catalysis is due to the formation of an acid via the ion exchange of the cation with the protons of the zeolite. The acid liberated was quantified using both titration experiments and ion-exchange experiments using sodium-exchanged zeolites. (C) 2004 Elsevier Inc. All rights reserved.