From microorganisms to humans, ferritin plays a central role in the biological management of iron. The ferritins function as iron storage and detoxification proteins by oxidatively depositing iron as a hydrous ferric hydroxide mineral core within their shell-like structures. The mechanism by which the mineral core is formed has been the subject of intense investigation for many years. A diiron ferroxidase site located on the H-chain subunit of vertebrate ferritins catalyzes the oxidation of Fe(II) to Fe(III) by molecular oxygen. A previous stopped-flow kinetics study of a transient μ-peroxodise(III) intermediate formed at this site revealed very unusual kinetics curves, the shape of which depended markedly on the amount of iron presented to the protein.(17) In the present work, a mathematical model for catalysis is developed that explains the observed kinetics. The model consists of two sequential mechanisms. In the first mechanism, turnover of iron at the ferroxidase site is rapid, resulting in steady-state production of the peroxo intermediate with continual formation of the mineral core until the available Fe(II) in solution is consumed. At this point, the second mechanism comes into play whereby the peroxo intermediate decays and the ferroxidase site is postulated to vacate its complement of iron. The kinetic data reveal for the first time that Fe(II) in excess of that required to saturate the ferroxidase site promotes rapid turnover of Fe(III) at this site and that the ferroxidase site plays a role in catalysis at all levels of iron loading of the protein (48-800 Fe/protein). The data also provide evidence for a second intermediate, a putative hydroperoxodise(III) complex, that is a decay product of the peroxo intermediate.