Abstract: Hydroxylation of phenol is prone to the production of tarry by-products. Selective hydroxylation of phenol can be obtained by reacting a limited amount of hydrogen peroxide with phenol in solution in a compatible organic solvent and in the presence of a catalyst that is at least partly soluble in the reaction medium and is the salt of a heteropolyacid of general formula: i) Q.sub.3 PMo.sub.m W.sub.12-m O.sub.40 or ii) Q.sub.3+v PM.sub.n V.sub.v O.sub.40, in which Q represents a compatible organic cation, m is zero or an integer less than 6, M represents molybdenum or tungsten, v is an integer which is up to 3, and n is an integer such that n+v=12. A preferred organic cation comprises cetyl pyridinium. Selectivity towards catechol is particularly observed employing heteropolyacid salts in which m=0 in formula i) and when n=11 and M=tungsten in formula ii) and towards hydroquinone when n=11 and M=molybdenum in formula ii). Preferably the reaction medium comprises acetonitrile.