The electron transport protein ferredoxin II (Fd II) from Desulfovibrio gigas contains an iron-sulfur cluster with an Fe3S4 core. We have studied the protein in the reduced state (cluster spin 5 = 2) with Mossbauer spectroscopy between 1.3 and 210 K in magnetic fields up to 6.0 T. Below 20 K one iron site of the cluster is Fe3+ whereas the other two sites form a delocalized Fe2+/Fe3+ pair. The itinerant electron is evenly delocalized over the pair. X-band EPR at 9 K reveals a “Am = 4” transition between two levels of the spin quintet. The highly resolved Mossbauer spectra were analyzed with an 5 = 2 spin Hamiltonian; 16 electronic and hyperfine parameters were determined with good precision. The positive magnetic hyperfine coupling constant Au = +15.6 MHz of the Fe3+ site shows that its local spin is antiparallel coupled to the system spin, and its magnitude suggests a spin of 9/2 for the delocalized dimer. The data were analyzed with a spin coupling model which takes into account Heisenberg exchange and valence delocalization. The description of the latter is gleaned from the Anderson-Hasegawa theory of double exchange. The effective Hamiltonian proposed here describes the data very well, and it holds promise for the description of valence-delocalized clusters with Fe4S4 cores. Above 20 K an additional spectral component appears in the Mossbauer spectra, suggesting population of an excited state with electron delocalization over all three sites. The transition rate between the ground-state and excited-state configuration is slow on the Mossbauer time scale (-10-7 s). Such a situation has not yet been reported for any iron-sulfur cluster.