The structures of two polymorphs of (mu-oxo)bis[(octaerhylporphinato)iron(III)], [Fe(OEP)]O-2 (C72H88Fe2N8O), have been determined by X-ray structure analyses using data collected with an area detector. The two [Fe(OEP)]O-2 polymorphs (a triclinic and a monoclinic form) show strong structural similarities. The eight Fe-Np bond distances have average values of 2.077(3) Angstrom in the triclinic form and 2.080(5) ii in the monoclinic form. The average axial Fe-O bond lengths are 1.756(3) and 1.755(10) Angstrom for the two structures, respectively. The average displacements of the iron(III) atom from the mean porphinato core are 0.50 Angstrom in the triclinic form and 0.54 Angstrom in the monoclinic form. The most important structural feature of the molecule in the two crystal forms is the near-eclipsed intramolecular porphyrin ring orientation : the average N-Fe-Fe’-N’ dihedral angles are 17.0(10)degrees in the triclinic form and 16.8(6)degrees in the; monoclinic form. Molecular mechanics calculations have been carried out to explore the inter-ring orientation; these suggest that peripheral ethyl group interactions lead to the near-eclipsed orientation. Crystal data : triclinic form, a = 11.561(5) Angstrom, b = 12.299(8) Angstrom, c = 23.341(16) Angstrom, alpha = 82.56(3)degrees, beta = 81.94(7)degrees, gamma = 79.18(1)degrees, space group P ($) over bar 1, V = 3210(5) Angstrom(3), Z = 2, 8931 observed data, R(1) = 0.050, R(2) 0.056; monoclinic form, a = 18.433(13) Angstrom, b = 15.104(6) Angstrom, c = 23.489(6) Angstrom, beta = 97.82(1)degrees, space group P2(1)/c, V = 6479(9) Angstrom(3), Z = 4, 6169 observed data, R(1) = 0.054, R(2) = 0.060.