Heteropoly acids (HPAs) are active catalysts for the Fries rearrangement of aryl esters (phenyl acetate, phenyl benzoate, and p-tolyl acetate) to yield the acylated phenols and esters together with phenols in homogeneous or heterogeneous liquid-phase systems at 100-170degreesC. Amongst the HPA catalysts studied are bulk, silica-supported, and sol-gel silica-included H3PW12O40 (PW), as well as bulk Cs+ and Ce3+ salts, CS2.5H0.5PW12O40 and Ce0.87H0.4PW12O40. The reaction with bulk and silica-supported PW occurs homogeneously in aryl esters (without solvent) or in polar solvents, such as nitrobenzene and o-dichlorobenzene. In non-polar solvents such as dodecane, the reaction is heterogeneous. The Cs+ and Ce3+ salts and sol-gel PW catalysts perform heterogeneously in all these media. From heterogeneous systems, the catalysts can be separated and reused, albeit with reduced activity. The homogeneous and silica-supported PW are much more active than H2SO4 and H-Beta zeolite, which is explained by the greater acid strength of HPA compared to the other acids. In contrast to silica-supported PW, the sol-gel PW catalysts show only a negligible activity in the Fries reaction of phenyl acetate, yielding mainly phenol with 92-100% selectivity. This may be explained by a weaker acid strength of the sot-gel catalysts and the presence of relatively high amount of water. P-31 MAS NMR shows that the state of the PW anion in the sol-gel and silica-impregnated catalysts is similar, but different from that in the bulk PW. (C) 2002 Elsevier Science B.V. All rights reserved.