Alkylperoxyl and arylperoxyl radicals were produced by pulse radiolysis in aqueous solutions, and their reactions with ferrous and ferrocyanide ions were studied by kinetic spectrophotometry. Oxidation of Fe(CN)(6)(4-) took place with rate constants that varied from <1 x 10(5) to 5 x 10(7) L mol(-1) s(-1), depending on the electron-withdrawing effects of the substituents on the peroxyl radical and presumably reflecting variations in reduction potential of the peroxyl radical, as expected for outer-sphere electron transfer. Oxidation of Fe-aq(2+), on the other hand, took place by an inner-sphere mechanism controlled by the rate of dissociative interchange of the water ligand. The rate constants were nearly the same for all the peroxyl radicals examined (k = (0.5-1.1) x 10(6) L mol(-1) s(-1)) and involved the formation of a transient intermediate, RO(2)(-)Fe(3+), which later decomposed to yield Fe-aq(3+). The decomposition was accelerated by H+ and by Fe2+. The proposed mechanism is a modification of a previously suggested reaction scheme.