A new unsymmetrical (μ-oxo)diiron(III) complex [Fe2O(OAsPh3)4Cl3]2[Fe2OCl6].2CH3CN (I), where OAsPh3 is triphenylarsine oxide, has been prepared by several ways and characterized by X-ray structural analysis, elemental analysis, spectral and magnetic methods. The complex (I) is formed simultaneously with another two complexes of the composition [Fe2O(OAsPh3)4Cl3][FeCl4].CH3CN (II) and [Fe(OAsPh3)4Cl2] [ FeCl4] (III). In compound (I) the complex cation has one Fe atom coordinated by four OAsPh3 which form a plane, and the oxo-bridged ligand occupies an apex and form a square pyramid configuration about the second iron atom. In the complex anion each Fe atom is tetrahedrally coordinated by three chlorine atoms and the oxo bridge. The magnetic properties have been investigated in the 4.2–300 K temperature range and these data correspond to the EPR spectra.