The kinetics of the Dushman reaction, 5I(-) + IO3- + 6H(+) --> 3I(2) + 3H(2)O, is considerably more complex in aqueous, acidic media than is the behavior of the analogous reaction of Br-with BrO3-. The latter reaction is first order in both [Br-] and [BrO3-] 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 H2Br2O3. In contrast, the [I-] order in the Dushman reaction is reported variously to be one or two, depending upon experimental conditions, especially [IO3-](0)/[I-](0), and the kinetics of this reaction is generally very complex with the intermediacy of H2I2O3 having been suggested by several authors. Stopped-flow kinetic measurements of the Dushman reaction are reported here with excess [KIO3](0) approximate to 40[KI](0)/5 and [HClO4](0) greater than or equal to 140 x 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 [KIO3](0) and [HClO4](0) increase. This observation is rationalized by a mechanism assuming the intermediacy of H2I2O3 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 H2I2O3 is a kinetically and stoichiometrically significant species in the Dushman reaction, while H2Br2O3 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.