The reactions of (OH)-O-. radicals with 2,4-dichlorophenoxyacetic acid (2,4-D), k = (6.6 +/- 0.5) x 10(9) M-1 s(-1), phenoxyacetic acid (PAA), k = (10 +/- 1) x 10(9) M-1 s(-1), and 2,4-dichlorophenol (2,4-DCP), k = (7.1 +/- 0.5) x 10(9) M-1 s(-1), have been studied using pulse and gamma radiolysis. The (OH)-O-. adducts produced by addition to the ring at positions not occupied by substituted groups were oxidized by K-3[Fe(CN)(6)], with the corresponding hydroxylated species analyzed by HPLC. The distribution of (OH)-O-. radical addition to the aromatic ring of 2,4-D is Cl, 17%; C2/C4, 20%; C3/C5/C6, 47%; and for (OH)-O-. reaction with PAA, it is Cl, 5.5%; C2/C6, 45%; C3/C5, 11%; C4, 36%. Whereas the hydroxycyclohexadienyl radicals of 2,4-D have no observable reactivity toward oxygen, those of PAA and 2,4-DCP do react, with k((OH)-O-. adduct-PAA) + O-2 = (5.1 +/- 0.2) x 10(8) M-1 s(-1) and k((OH)-O-. adduct-2,4-DCP) + O-2 = (1.1 +/- 0.1) x 10(8) M-1 s(-1). The phenoxyl radicals formed by chloride elimination from the ipso-chloro-(OH)-O-. adducts of 2,4-D rapidly oxidize TMPD to TMPD.+, k = (3.6 +/- 0.5) x 10(9) M-1 s(-1).