Acid treatment of the mu-oxo bridge in (mu-oxo)(octaethylporphinato)iron(III) leads to [Fe(OEP)(H2O)]ClO4 or {[Fe(OEP)]2(OH)}ClO4 rather than the expected perchlorato or bis(aquo) species. Synthetic procedures, the X-ray structure determination, and Mossbauer and magnetic susceptibility measurements for aquo(octaethylporphinato)iron(III) perchlorate, [Fe(OEP)(H2O)]ClO4.2H2O are described. The Mossbauer spectrum (at 4.2 K) shows a quadrupole doublet with a large splitting of DELTAE(q) = 3.287(5) mm/s and an isomer shift of 0.391(5) mm/s. The temperature-dependent magnetic susceptibility data are understood in terms of an admixed intermediate-spin state (S = 3/2, 5/2) and are readily fit to a Maltempo model with two interacting iron(III) Centers and parameters g perpendicular = 4.65, zeta = 150 cm-1, and J = -1.0 cm-1. This dimeric model is consistent with the low-temperature (T = 124 K) crystal structure which reveals [Fe(OEP)(H2O)]22+ units. These are formed by a pi-pi interaction between cations with an Fe ... Fe distance of 5.08 angstrom, a lateral shift of 3.39 angstrom, and an interplanar spacing of 3.39 angstrom between the mean planes of the cores. These dimeric units are further linked together into an infinite linear chain by a hydrogen bond network involving the axial aquo ligands, the perchlorate anions, and two additional water molecules per iron porphyrin unit. In the Fe(OEP)(H2O)]+ cation, the average Fe-N(p) distance is 1.982(4) angstrom, the axial Fe-O distance is 2.045(3) angstrom, and the iron atom is displaced 0.20 angstrom from the 24-atom mean plane. These structural data are consistent with an admixed intermediate-spin state for iron(III). Crystal data for [Fe(OEP)(H2O)]ClO4.2H2O : T = 124 +/- 2 K, a = 12.169(10) angstrom, b = 13.388(11) angstrom, c = 13.409(10) angstrom, alpha = 62.71(6)degrees, beta = 88.65(6)degrees, gamma = 67.97(6)degrees, V = 1770.2(2.5) angstrom3, triclinic, space group P1BAR, Z = 2, R1 = 0.046, and R2 = 0.064 for 4817 observed data. In attempted syntheses of a diaquo complex, we have isolated and structurally characterized the known compound [Fe(OEP)(THF)2]ClO4; more precise structural results are reported. Crystal data for [Fe(OEP)(THF)2]ClO4 : T = 292 +/- 1 K, a = 13.918(1) angstrom, b = 16.519(6) angstrom, c = 10.654(2) angstrom, beta = 118.21(2)degrees, V = 2158.5(5.3) angstrom3, monoclinic, space group C2/m, Z = 2, R1 = 0.054, and R2 = 0.059 for 2263 observed data.