The synthesis and characterization are reported of two new polynuclear Fe-III complexes containing the anion of 8-hydroxyquinoline (hqnH), an N,O-chelating ligand. The complexes are [Fe8O4(O2CPh)(10)(hqn)(4)(OMe)(2)] (1) and [Fe6O2(OH)(2)(O2CPh)(10)(hqn)(2)] (2) and were obtained from reactions in MeOH (1) or H2O (2) using either low-nuclearity preformed clusters or simple metal salts as starting materials. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out and indicate S = 0 and S = 5 ground states for 1 and 2, respectively. In order to rationalize the ground states of these and other higher-nuclearity Fe-III/O clusters, a magnetostructural correlation (MSC) has been developed specifically for polynuclear Fe-III/O systems that predicts the exchange interaction constant (J(ij)) between two Fe-III atoms based on the Fe-O distances and Fe-O-Fe angles at monoatomically bridging ligands. This correlation was refined using selected tri- and tetranuclear complexes in the literature for which both crystal structures and reliable experimentally determined J(ij) values were available. The predictive capability of the MSC was evaluated by rationalising the ground-state spins of 1, 2, and other Fe-5-Fe-8 clusters, simulating the dc magnetic susceptibility data of polynuclear Fe-III complexes, and fitting experimental dc magnetic susceptibility vs T data. The latter fits were evaluated to identify and eliminate systematic errors, and this allowed a protocol to be developed for application of this MSC to other polynuclear Fe-III/oxo clusters.