The kinetics and mechanisms of ligand substitution reactions of the iron(III) hydroxo dimer, Fe-2(mu-OH)(2)(H2O)(8)(4+), with various inorganic ligands were studied by the stopped-flow method at 10.0 or 25.0 degreesC in 1.0 M NaClO4. The transient formation of the following di- and tetranuclear complexes was confirmed: Fe-2(OH)SO43+, Fe-2(OH)H2PO24+, Fe-2(OH)HPO33+, Fe-2(OH)SeO33+, and Fe-4(AsO4)(OH)(2)(7+). The catalytic effect of arsenic(III) on the hydrolytic reaction of iron(III) was also attributed to the formation of a dinuclear complex at very low concentration levels. Fast formation and subsequent dissociation of the multinuclear species into the corresponding mononuclear complexes (FeL) proceed via parallel reaction paths which, in general, show composite pH dependencies. The appropriate rate laws were established. The reactions of the different ligands occur at very similar rates, though the uninegatively charged singly deprotonated form reacts about 1 order of magnitude faster than the neutral form of the same ligand. The results can conveniently be interpreted in terms of a dissociative interchange mechanism which postulates the formation of an intermediate complex in which the ligand is coordinated to only one Fe(III) center of the hydroxo dimer. In a subsequent fast step, the ligand forms a bridge between the two metal ions by replacing one of the OH groups. The dissociation of the dinuclear complex into FeL most likely proceeds via the same intermediate.