Molecular oxygen is produced from water via the following reaction of potassium ferrate (K2FeO4) in acidic solution: 4[H3Fe(VI)O4](+) + 8H3O(+) → 4Fe(III)(3+) + 3O2 + 18H2O. This study focuses upon the mechanism by which the O-O bond is formed. Stopped-flow kinetics at variable acidities in H2O and D2O are used to complement the analysis of competitive oxygen-18 kinetic isotope effects (18O KIEs) upon consumption of natural abundance water. The derived 18O KIEs provide insights concerning the identity of the transition state. Water attack (WA) and oxo-coupling (OC) transition states were evaluated for various reactions of monomeric and dimeric ferrates using a calibrated density functional theory protocol. Vibrational frequencies from optimized isotopic structures are used here to predict (18)O KIEs for comparison to experimental values determined using an established competitive isotope-fractionation method. The high level of agreement between experimental and theoretic isotope effects points to an intramolecular OC mechanism within a di-iron(VI) intermediate, consistent with the analysis of the reaction kinetics. Alternative mechanisms are excluded based on insurmountably high free energy barriers and disagreement with calculated 18O KIEs.