Detailed studies of oxygen atom exchange (OAE) between H2 18O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the FeIV complex, which is evidenced by OAE rates that are linearly dependent on [H2 18O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2 18O] and much larger H2O/D2O KIEs of 5-6. [FeIV(O)(TMC)(MeCN)]2+ (1) and [FeIV(O)(MePy2TACN)]2+ (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [FeIV(O)(N4Py)]2+ (3), [FeIV(O)(BnTPEN)]2+ (4), [FeIV(O)(1Py-BnTPEN)]2+ (5), [FeIV(O)(3Py-BnTPEN)]2+ (6), and [FeIV(O)(Me2Py2TACN)]2+ (8) represent complexes that fall in the second kinetic regime. Interestingly, [FeIV(O)(PyTACN)(MeCN)]2+ (7) exhibits a linear [H2 18O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2 18O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.