The kinetics and mechanism of the iron(III)-sulfite ion reaction were studied by using the stopped-flow method at 10.0 degrees C in 1.0 M NaClO4. The formation of a 1:1 and a 2:1 complex was confirmed in the initial phase of the reaction when the metal ion was in excess over the ligand. In this system the formation of the dimer complex, Fe2SIV, was not reported before. The following composition is proposed for the new species : (H2O)(4)Fe(SO3-)(OH)Fe(H2O)(4)(3+). The stability constant of the complex formation reaction, Fe-2(OH)(2)(4+) + HSO3- reversible arrow Fe2SIV was determined from absorbance data at 430 nm : log K-21 = 3.37 +/- 0.16. According to detailed kinetic studies the complex formation is first order in both Fe-2(OH)(2)(4+) and sulfite ion. The pH dependence of the measured rate constants indicates two kinetically significant pathways with the SO32- and HSO3- forms of the ligand, respectively. The corresponding rate constants are the following : k(HSO3)(-) = (4.5 +/- 0.3) x 10(4) M-1 s-(1) and kSO(3)(2-) = (2.1 +/- 0.2) x 10(9) M-1 s(-1) Mechanistic aspects of the complex formation are discussed in detail.