The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4''-t-butylpyridinium))porphyrinato]iron(III) octabromide, , with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and NMR spectroscopy and NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of indicated a value of 5.0 for coordinated water in . The monohydroxo-ligated formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, , significantly faster than water exchange on , viz. k_{ex}=5.5 x 10^{4}s^{-1} at . The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, . The diaqua-ligated binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other complexes. Coordination of NO to at high pH follows an associative mode, as evidenced by negative S⧧ and V⧧ values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate to is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these complexes as a function of pH is discussed on the basis of available experimental data.