The mixed-valent polyiron oxo complex [FeIII4FeIIs(O)2(OMe)18(OAc)6(MeOH)4.67] (1) was prepared through the controlled oxidation of a methanol solution of ferrous acetate and lithium methoxide. The structure of 1, revealed in a single-crystal X-ray diffraction experiment, consists of a face-centered cubic array of oxygen atoms with iron(II) and iron(III) ions in the octahedral interstices. Two mu6-OXO ligands, to which ten of the twelve iron atoms are coordinated, form the core of the structure. The iron atoms have distorted octahedral coordination geometries with oxo, methoxide, methanol, and acetate ligands. From an analysis of the Fe-O bond lengths and from charge considerations, four of the twelve iron atoms were assigned as ferric ions. Crystals of 1 are air-sensitive and lose coordinated methanol when removed from the reaction solution. The mixed-valent nature of 1 was further characterized by electronic spectroscopy. There is a broad feature at 694 nm with shoulders at 900 and 1250 nm attributable to iron(II) ligand field and intervalence charge transfer transitions. The iron atoms of 1 exhibit overall antiferromagnetic exchange coupling, with chi(T) decreasing from 40.3 emu mol-1 K at 300 K to 7.04 emu mol-1 K at 2.5 K. The calculated spin-only chi(T) value for four iron(III) ions with g = 2.0 and eight iron(II) ions with g = 2.2 is 46.5 emu mol-1 K. No saturation was observed in a high-field magnetization study of 1 at 1.4 K, the maximum value attained being 18.2 mu(B) at 20.0 T. Mossbauer spectral studies of 1 at 20 K revealed three distinguishable quadrupolar doublets, one with spectral parameters characteristic of iron(III) (delta = 0.47 and DELTAE(Q) = 0.74 mm s-1) and two quadrupolar resonances arising from iron(II) ions (delta = 1.28 and 1.29 and DELTAE(Q) = 2.02 and 3.29 mm s-1, respectively). The relative absorption areas for the three spectral features were consistent with the foregoing assignment of oxidation states. No valence delocalization was observed for 1 up to 250 K. Below 20 K, intermediate relaxation phenomena were evident for all three quadrupole doublets.