The crystal structure of the title compound, (OH2)3(phen)FeOFe(phen)(OH2)34.(H2O)(phen = 1,10-phenanthroline), has been determined by single-crystal X-ray diffraction methods at 295 K, being refined by least-squares methods to a residual of 0.059 for 3467 independent 'observed'reflections. Crystals are monoclinic, P 21/c, a 18.202(6), b 9.771(4), c 19.459(7) Ŭ ߠ94.46(3)ꬠZ = 4. Two of the four anions are disordered. Within the cation, the geometry is typical of high-spin iron(III). O(bridge)-Fe are 1.765(4), 1.784(4) Ŭ with Fe-O-Fe 162.0(3)ꮠThe three water molecules in the coordination sphere of each iron atom are fac; those trans to the O-Fe bond have long Fe-(OH2) distances [2.154(5), 2.125(5) ŝ compared to those trans to the phenanthroline chelates [2.028(5)-2.037(5) ŝ. Fe-N range from 2.143(6) to 2.166(6) Ů