Structural changes accompanying the reduction of (mu-oxo)bis[(meso-tetrakis(methoxyphenyl)porphyrinato)-iron], [Fe(TMPP)]O-2, irreversibly adsorbed on Black Pearls 2000 (BP) high area carbon in aqueous electrolytes have been examined in situ by X-ray absorption fine structure (XAFS). In the pH range 5-10.8, the average iron-to-porphinato nitrogen distance, d(Fe-N-p), for the ferric species, using theoretical phases and amplitudes calculated from FEFF 3.28 (2.09 +/- 0.01 Angstrom), was found to be, within experimental error, the same as that reported for the closely related [Fe(TPP)]O-2 in crystalline form (2.087 Angstrom). At extreme pH values d(Fe-N-p) was smaller (2.04 +/- 0.02 Angstrom for pH 1.2 and 3.1 and 2.05; +/- 0.02 Angstrom for pH = 13) than those observed in the intermediate PH range. The values in strong acid are consistent with those reported for the diaquo axially coordinated complex [Fe(TPP)(OH2)(2)](+) (d(Fe-N-p) = 2.045 +/- 0.005 Angstrom), whereas that at pH 13, d(Fe-N-p) = 2.05 +/- 0.02, strongly suggests an axially coordinated dihydroxy complex as the predominant species. :The latter assignment is in agreement with information derived from other Spectroscopic methods for closely related ferric porphyrins in solution phase. In contrast, the corresponding ferrous counterparts displayed values for d(Fe-N-p) (2.02 +/- 0.02 Angstrom) consistent with the iron center placed in the plane of the ring throughout the whole range of pH values. Insight into the possible axial ligation of the ferrous derivatives was : obtained from the dependence of the potential associated with the voltammetric peaks on the solution PH. Spin-state/ stereochemical relationships derived from X-ray crystallography and magnetic measurements of well-characterized iron porphyrins indicate that the adsorbed ferric species are all high spin, whereas the ferrous counterparts are either high or intermediate spin.