((FeO)-O-IV](2+) species have been implicated as the active form of many nonheme iron enzymes. The electronic structures of iron(IV) oxo complexes are thus of great interest. High-frequency and -field electron paramagnetic resonance is employed to determine accurately the spin Hamiltonian parameters of two stable complexes that contain the Fe=O unit: [FeO(TMC)(CH3CN)](CF3SO3)(2), where TMC = tetramethylcyclam and [FeO(N4py)](CF3SO3)(2), where N4Py = bis(2-pyridylmethyl)bis(2-pyridyl)methylamine. Both complexes exhibit zero-field splittings that are positive, almost perfectly axial, and of very large magnitude: D = +26.95(5) and +22.05(5) cm(-1), respectively. These definitive experimental values can serve as the basis for further computational studies to unravel the electronic structures of such complexes.