A detailed study of the effect of pH, temperature, and pressure on the reaction of hydrogen peroxide with [Fe-III(p(8-))](7-), where P8-represents the octa anionic porphyrin, was performed using stopped-flow techniques. Depending on the pH, different high valent iron-oxo species were formed. At pH < 9 formation of a two-electron oxidized species [(porphyrin(+center dot))Fe-IV=O] was observed. In contrast, at pH > 9 only the one electron oxidized species [(porphyrin)Fe-IV=O] was found to be present in solution. Under selected conditions at pH 8 it was possible to determine rate constants for both the coordination of hydrogen peroxide and subsequent heterolytic cleavage of the O-O bond. At pH 11 a composite rate constant for coordination of H2O2 and homolytic cleavage of the O-O bond could be measured, In addition, it was possible to determine the activation parameters for the overall reaction sequence leading to the formation of [(porphyrin)Fe-IV=O]. Careful analysis of the obtained data supports an associatively activated mechanism for the coordination of hydrogen peroxide. The catalytic properties of [Fe-III(p(8-))](7-) in the presence of H2O2 were also investigated. Both high valent iron-oxo species turned out to be able to oxidize diammonium-2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) to the radical cation ABTS(+center dot). At higher hydrogen peroxide concentrations a reduced yield of ABTS(+center dot) was observed because of increased catalase activity of [Fe-III(p(8-))](7-). At high pH disproportionation of ABTS(+center dot) to ABTS and ABTS(2+) occurred, which could be suppressed by an excess of unreacted ABTS. In slightly basic to acidic solutions this reaction did not play a role.