High-frequency EPR data are reported for the Fe(II/III) valence delocalized dinuclear complex [Fe-2(OH)(3)-(tmtacn)(2)](2+). A full-matrix diagonalization approach is used to derive the spin-Hamiltonian parameters for this S-T = 9/2 complex. At high fields (up to 14.5 T) and high frequencies (189-433 GHz) fine structure peaks due to resonances between the Kramers doublets (M-s = 9/2, 7/2,...) are observed. The spacing of the fine structure reveals that the axial zero-field splitting (ZFS) parameter D is +1.08(1) cm(-1); a very small rhombic ZFS (\E\ less than or equal to 0.01 cm(-1)) is suggested by line broadening of these interdoublet resonances. Simulations reveal that g is close to 2.00, and very nearly isotropic: g(x) = g(y) = g(z) = 2.00(2). This complex is a model for the valence-delocalized [Fe2S2](+) pairs found in larger iron-sulfur clusters, such as the cofactors from the nitrogenase system. This work indicates that HFEPR is a viable technique for the study of high-spin centers in proteins.