The polyanionic water-soluble and non-mu-oxo-dimer-forming iron porphyrin iron(III) 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(2),5(6),15(2),15(6)--tetrakis[2,2-bis(carboxylato)ethyl]-5,10,1 5,20-tetraphenylporphyrin, (P8-)Fe-III (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV-vis and H-1 NMR spectroscopy. A single pK(a1) value of 9.26 was determined for the deprotonation of coordinated water in (p(8-))Fe-III(H2O)(2) (1-H2O) present in aqueous solution at pH < 9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P8-)Fe-II(NO+)(L), where L = H2O or OH-. The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters Delta S-double dagger and Delta V-double dagger for the "on" and "off" reactions. The rate constant k(on) = 6.2 x 10(4) M-1 s(-1) (24 degrees C), determined for NO binding to the monohydroxo-ligated (P8-)Fe-III(OH) (1-OH) present in solution at pH > 9, is markedly lower than the corresponding value measured for 1-H2O at lower pH (k(on) = 8.2 x 10(5) M-1 s(-1), 24 degrees C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H2O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure.