The rate of Fe3+ release from horse spleen ferritin (HoSF) was measured using the Fe3+-specific chelator desferoxamine (DES). The reaction consists of two kinetic phases. The first is a rapid non-linear reaction followed by a slower linear reaction. The overall two-phase reaction was resolved into three kinetic events: 1) a rapid first-order reaction in HoSF (k1); 2) a second slower first-order reaction in HoSF (k 2); and 3) a zero-order slow reaction in HoSF (k3). The zero-order reaction was independent of DES concentration. The two first-order reactions had a near zero-order dependence on DES concentration and were independent of pH from 6.8 to 8.2. The two first-order reactions accounted for 6-9 rapidly reacting Fe3+ ions. Activation energies of 10.5 ± 0.8, 13.5 ± 2.0 and 62.4 ± 2.1 kJ/mol were calculated for the kinetic events associated with k1, k2, and k3, respectively. Iron release occurs by: 1) a slow zero-order rate-limiting reaction governed by k3 and corresponding to the dissociation of Fe3+ ions from the FeOOH core that bind to an Fe3+ binding site designated as site 1 (proposed to be within the 3-fold channel); 2) transfer of Fe3+ from site 1 to site 2 (a second binding site in the 3-fold channel) (k2); and 3) rapid iron loss from site 2 to DES (k1).