A conserved iron-binding site, the ferroxidase center, regulates the vital iron storage role of the ubiquitous protein ferritin in iron metabolism. It is commonly thought that two Fe(II) simultaneously bind the ferroxidase center and that the oxidized Fe(III)-O(H)-Fe(III) product spontaneously enters the cavity of ferritin as a unit. In contrast, in some bacterioferritins and in archaeal ferritins a persistent di-iron prosthetic group in this center is believed to mediate catalysis of core formation. Using a combination of binding experiments and isotopically labeled (57)Fe(II), we studied two systems in comparison: the ferritin from the hyperthermophilic archaeal anaerobe Pyrococcus furiosus (PfFtn) and the eukaryotic human H ferritin (HuHF). The results do not support either of the two paradigmatic models; instead they suggest a unifying mechanism in which the Fe(III)-O-Fe(III) unit resides in the ferroxidase center until it is sequentially displaced by Fe(II).