Proton ENDOR spectroscopy on gamma-irradiated single crystals of the synthetic model compound (Et4N)(2)-[Fe4S4(SC6H4-o-OH)(4)] has been used to study a [4Fe-4S](+) center that has properties similar to those of the reduced states of some particular ferredoxins and of the enzyme aconitase. The interesting point about this model compound is that one iron atom is five-coordinate, with an extra phenolic oxygen attached to one of its thiolate ligand. The complex thus represents an example of an asymmetrical [4Fe-4S] cluster in the solid crystalline state. From the study of the angular dependence of the proton ENDOR lines, it has been possible to derive eight hyperfine tensors of protons occupying different positions in the near vicinity of the iron atoms, the main bearers of the spin population of the paramagnetic anion. Some protons belong to thiolate ligands of the anion whereas others belong to close (Et4N)(+) counterions. From the anisotropic parts of the tensors-and by using a multicentric point-dipole model-it has been possible to derive the distribution of the unpaired electron spin population on the different iron (and sulfur) atoms of the cluster. The spin populations thus obtained indicate that two iron atoms (Fe-2 and Fe-3) constitute a localized Fe3+-Fe2+ mixed-valence pair. Within the limitations of a pairwise vectorial spin coupling model that considers two iron pairs, these results show that the magnetic ground state of the center studied here most probably corresponds to a \9/2, 4, 1/2] spin state, where 9/2, 4, and 1/2 are the spin quantum numbers of the mixed-valence pair, the ferrous pair, and the cluster, respectively. Finally, it is found that the spin populations on the different iron atoms, obtained for this model system, are quite similar to those of reduced substrate-free aconitase derivable from previous Mossbauer and ENDOR measurements.