The complex [iron(III) (octaphenylsulfonato)porphyrazine](5-), Fe-III(Pz), was synthesized. The pK(a) values of the axially coordinated water molecules were determined spectrophotometrically and found to be pK(a1) = 7.50 +/- 0.02 and pK(a2) = 11.16 +/- 0.06. The water exchange reaction studied by O-17 NMR as a function of the pH was fast at pH = 1, k(ex) = (9.8 +/- 0.6) x 10(6) s(-1) at 25 degrees C, and too fast to be measured at pH = 10, whereas at pH = 13, no water exchange reaction occurred. The equilibrium between mono- and diaqua FeIII(Pz) complexes was studied at acidic pH as a function of the temperature and pressure. Complex-formation equilibria with different nucleophiles (Br- and pyrazole) were studied in order to distinguish between a five- (in the case of Br-) or six-coordinate (in the case of pyrazole) iron(Ill) center. The kinetics of the reaction of Fe-III(Pz) with NO was studied as a model ligand substitution reaction at various pH values. The mechanism observed is analogous to the one observed for iron(Ill) porphyrins and follows an Id mechanism. The product is (Pz)(FeNO+)-N-II, and subsequent reductive nitrosylation usually takes place when other nucleophiles like OH- or buffer ions are present in solution. Fe-III(Pz) also activates hydrogen peroxide. Kinetic data for the direct reaction of hydrogen peroxide with the complex clearly indicate the occurrence of more than one reaction step. Kinetic data for the catalytic decomposition of the dye Orange 11 by H2O2 in the presence of Fe-III(Pz) imply that a catalytic oxidation cycle is initiated. The peroxide molecule first coordinates to the iron(III) center to produce the active catalytic species, which immediately oxidizes the substrate. The influence of the catalyst, oxidant, and substrate concentrations on the reaction rate was studied in detail as a function of the pH. The rate increases with increasing catalyst and peroxide concentrations but decreases with increasing substrate concentration. At low pH, the oxidation of the substrate is not complete because of catalyst decomposition. The observed kinetic traces at pH = 10 and 12 for the catalytic cycle could be simulated on the basis of the obtained kinetic data and the proposed reaction cycle. The experimental results are in good agreement with the simulated ones.