The kinetics and mechanism of the iron(III)-phosphate ion reaction were studied at large iron(III) excess using the stopped-flow method at 10.0 degrees C in 1.0 M NaClO4. In the first few hundred milliseconds of the reaction, the formation of a novel tetranuclear complex was confirmed. The following composition is proposed for the new species: Fe-4(PO4)(OH)(2)(H2O)(16)(7+). According to detailed kinetic studies, the formation of this species is first order with respect to Fe-2(OH)(2)(H2O)(8)(4+) and H2PO4- and presumably proceeds via a dinuclear intermediate Species. At longer reaction times slow dissociation of the tetranuclear complex controls the formation of the thermodynamically favored Fe(PO4)(H2O)(5) complex. The overall reaction was interpreted in terms of the following reactions: Fe-2(OH)(2)(H2O)(8)(4+) reversible arrow 2Fe(mn)(3+); Fe-2(OH)(2)(H2O)(8)(4+) + P(V) reversible arrow Fe2PV; Fe2PV + Fe-2(OH)(2)(H2O)(8)(4+) reversible arrow Fe(4)p(V); Fe-mn(3+) + P(V) reversible arrow Fe(PO4)(H2O)(5). (Fe-mn(3+) = Fe(H2O)(6)(3+) + Fe(OH)(H2O)(5)(2+); P(V) = H3PO4 + H2PO4-; Fe2PV = Fe-2(HPO4)(OH)(H2O)(8)(3+); Fe4PV = Fe-4(PO4)(OH)(2)(H2O)(16)(7+)) The pH dependence and relevant rate and equilibrium constants are reported for the individual reaction steps.