The mechanism of the acylation of anisole with (x,p-unsaturated acids, i.e. acrylic, crotonic and methylcrotonic acid, has been investigated using phosphotungstic acid (H3PW12O40), as pure form, supported on SiO2 and in the form of cesium salts as catalysts. Since alpha,beta-unsaturated acid can either alkylate and/or acylate the aromatic compound, the influence of the catalyst on the selectivity for these two competing reactions was studied. Analysis of products obtained on the acylation of aromatic compounds with alpha,beta-unsaturated acids shows that all the catalysts are more active for acylation than alkylation. Secondary products coming from intermolecular reactions of the acylated product with anisole as well as tertiary products coming from its further decomposition and recombination with another anisole molecule were observed. Heteropolyacids supported on silica were found to be more active and selective towards acylation reactions than zeolites HY and HP, even when the activity per acid site was considered. Zeolites and cesium salts from heteropolyacids favor acid polymerization reaction rather than the acylation of the aromatic compound. (C) 2002 Elsevier Science B.V. All rights reserved.