The kinetics of the Dushman reaction, 5I -+ IO 3 -+ 6H + f 3I 2 + 3H 2 O, is considerably more complex in aqueous, acidic media than is the behavior of the analogous reaction of Br -with BrO 3 -. The latter reaction is first order in both [Br -] and [BrO 3 -] and is believed to proceed via a series of simple oxygen-atom transfers, there being no evidence for the intermediacy of polybromine species such as H 2 Br 2 O 3 . In contrast, the [I -] order in the Dushman reaction is reported variously to be one or two, depending upon experimental conditions, especially [IO 3 -] 0 /[I -] 0 , and the kinetics of this reaction is generally very complex with the intermediacy of H 2 I 2 O 3 having been suggested by several authors. Stopped-flow kinetic measurements of the Dushman reaction are reported here with excess [KIO 3 ] 0 ≈ 40[KI] 0 /5 and [HClO 4 ] 0 g 140 × 6[KI] 0 /5 in which the observed [I -] order is cleanly two, but saturation kinetics is observed for the pseudo-second-order rate constant as [KIO 3 ] 0 and [HClO 4 ] 0 increase. This observation is rationalized by a mechanism assuming the intermediacy of H 2 I 2 O 3 at concentrations significant compared to those of I -, requiring that the kinetic equations be constrained by the iodide-ion mass balance. It is suggested that H 2 I 2 O 3 is a kinetically and stoichiometrically significant species in the Dushman reaction, while H 2 Br 2 O 3 is not in the analogous bromine system, because of the greater atomic radius, polarizability, and ability to expand its valence shell of iodine as compared to bromine.