Hydroxyl radicals generated by Fenton's reagent rapidly oxidize phenolic acids in aqueous solution. Analysis of second-order rate constants for correlations reveals quantitative structure-activity relationships (QSAR). The results were analysed so as to obtain the relation between the chemical structure of the substituted benzene ring and its reactivity with the hydroxyl radical. The rate constants for oxidation of selected phenolic acids, ranged from 1.24x10(9) M(-1)s(-1) for 3,4,5-trimethoxybenzoic acid to 2.57x10(9) M(-1)s(-1) for 3-(4-hydroxyphenyl)propionic acid, respectively, to provide good Hammett correlations. The results of the Hammett correlation are consistent with the expected reaction mechanism where the rate-limiting step is the initial hydroxylation of the phenolic compound and the formation of a hydroxycyclohexadienyl radical. (C) 2009 Elsevier B.V. All rights reserved.