The rate of Fe 3+ release from horse spleen ferritin (HoSF) was measured using the Fe 3+ -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 (k 1 ); 2) a second slower first-order reaction in HoSF (k 2 ); and 3) a zero-order slow reaction in HoSF (k 3 ). 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 Fe 3+ ions. Activation energies of 10.5±0.8, 13.5±2.0 and 62.4±2.1kJ/mol were calculated for the kinetic events associated with k 1 , k 2 , and k 3 , respectively. Iron release occurs by: 1) a slow zero-order rate-limiting reaction governed by k 3 and corresponding to the dissociation of Fe 3+ ions from the FeOOH core that bind to an Fe 3+ binding site designated as site 1 (proposed to be within the 3-fold channel); 2) transfer of Fe 3+ from site 1 to site 2 (a second binding site in the 3-fold channel) (k 2 ); and 3) rapid iron loss from site 2 to DES (k 1 ).