X-band EPR single crystal studies have been done on two Fe-O-Fe dimers, Na-4[Fe(edta)]O-2 . 3H(2)O and {[Fe-(phen)(2)]O-2}(NO3)(4) . 7H(2)O, and the room-temperature X-ray structure of Na-4[Fe(edta)]O-2 . 3H(2)O was obtained. The compound crystallizes as monoclinic in the space, group P2(1)/n (No. 14), having cell parameters a = 16.08(1) Angstrom, b = 10.776(4) Angstrom, c = 18.367(8) Angstrom, beta = 106.44(4)degrees, V = 3051(4) Angstrom(3), and Z = 4; R = 0.0490, R(w) = 0.0369. Resonances were identified by intensity variation with temperature for both the quintet (S = 2) and septet (S = 3) states of the dimer in both compounds and their spin-Hamiltonian parameters measured. A single resonance line for the triplet state was also observed in the phen complex in a limited range of orientations at a magnetic field near the limit of our magnet (15 kG). The Zero-field-splitting (ZFS) parameters obtained were D-2, E(2), D-3, E(3) = 0.248(1), 0.060(1), 0.604(1), -0.016(1) cm(-1) for the edta complex and 0.256(1), 0.037(1), 0.686(1), -0.008(1) cm(-1) for the phen complex. From these values, we can separate the contribution of the exchange parameter anisotropy from that due to local distortions of the ligand field near each Fe(III) ion. /D-exchange/, /E(exchange)/ = 1.25, 0.03 cm(-1) for the edta complex and 1.41, 0.02 cm(-1) for the phen complex, and in both complexes the z axis is parallel to the Fe-Fe axis. These values are surprisingly large, considering the theories presently used to explain the same effect in Cu-O-Cu dimers, and indicate that these theories need to be reexamined. Our results predict that /D-1/, /E(1)/ = 3.00, 0.62 cm(-1) for the edta complex and 2.33, 0.31 cm(-1) for the phen complex. Confirmation of these values will require EPR studies at frequencies greater than Q-band.