Molecular oxygen is produced from water via the following reaction of potassium ferrate (K2FeO4) in acidic solution: 4[(H3FeO4)-O-VI](+) + 8H(3)O(+) -> 4Fe(3+) + 3O(2) + 18H(2)O. 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 (O-18 KIEs) upon consumption of natural abundance water. The derived O-18 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 O-18 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 O-18 KIEs.