The oxidation of iodide ion by chlorine dioxide has been studied by stopped-flow techniques at I ) 1.0 M (NaClO 4 ). The following two-term rate law was confirmed for the reaction:The rate constants at 298 K and the activation parameters are k I ) (1.87 ( 0.02) × 10 3 M -1 s -1 , ∆H I q ) 35.4 ( 0.7 kJ/mol, ∆S I q ) -63.5 ( 2.3 J/(mol K), k II ) (1.25 ( 0.04) × 10 4 M -2 s -1 , ∆H II q ) 36.7 ( 1.3 kJ/mol, ∆S II q ) -43.2 ( 4.6 J/(mol K). Both the second-and third-order paths are interpreted in terms of an outersphere electron-transfer mechanism. The calculations based on the Marcus theory yield k I ) 1358 M -1 s -1 for the second-order path.