The reaction of t-BuOOH with the novel octacationic, water soluble, nonaggregating, and non-mu-oxo dimer forming meso-5,10,15,20-tetrakis(2,4,6-trimethylammoniummethyl)phenyl)porphyrinatoiron(III) dihydrate {(3)Fe-III(X)(2), where X = H2O and/or HO-} was studied in buffered aqueous solutions between pH 0.9 and 12.1 at 30 degrees C and at constant ionic strength of 0.20. Minimal catalyst turnover conditions with [t-BuOOH](initial) = 6.0 x 10(-5) M and [(3)Fe-III(X)(2)] = 1.9 x 10(-6) M were employed, in the presence and absence of excess 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) disodium salt (ABTS, with [ABTS](initial) = 6.0 x 10(-3) M). ABTS serves to trap any hypervalent iron-porphyrin species as well as alkoxyl radical products. The reaction was established to be first-order in (3)Fe-III(X)(2) and t-BuOOH concentrations. The large number of positive charges on (3)Fe-III(X)(2) bring about competitive inhibition of the reaction by oxyanionic buffers. No general acid or general base catalysis is observed at any pH. A plot of the logarithms of k(ly), (the apparent second-order rate constants obtained by extrapolation to zero buffer concentration at each pH), vs pH was fit to a kinetic equation which takes into account three productive intermediates (3)Fe-III(H2O)(t-BuOOH), (3)Fe-III(H2O)(t-BuOO(-)), and (3)Fe-III(OH-)(t-BuOO(-)). The major features of the pH-rate profiles of octacationic (3)Fe-III(X)(2) and tetraanionic tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphyriniron(III) hydrate (1)Fe-III(X)(2) are much alike. Shifting of the profile toward lower pH values for (3)Fe-III(X)(2) is consistent with lowered pK(a) values for (3)Fe-III(H2O)(2) when compared to (1)Fe-III(H2O)(2). pH-Dependent second-order rate constants for reaction of t-BuOOH with the octacationic (3)Fe-III(X)(2) and tetraanionic (1)Fe-III(X)(2) are not greatly different. A plot of log k(ly) at pH 5.2 vs pK(a) of ROH in the reactions of (3)Fe-III(X)(2) with various alkyl and acyl hydroperoxides (ROOH) shows a break at pK(a) similar to 6. At pK(a) (ROH) below 6, a large dependence of reaction rates on the pK(a) of ROH is observed (-beta(1g) similar to 1.0), indicative of a polar mechanism (heterolysis). Above pK(a) = 6, a slight dependence on polar effects (-beta(1g) similar to 0.2) suggests a less polar transition state. The products from the reaction of t-BuOOH with (3)Fe-III(X)(2) are discussed in terms of four mechanisms : (a) heterolysis of t-BuOOH with a contribution from radical chain reactions; (b) a radical chain mechanism initiated by (3)Fe-IV(X)(O); (c) Fenton chemistry involving (3)Fe-II and (3)Fe-III states; or (d) rate-limiting homolysis of the O-O bond in t-BuOOH.