The oxidation of iodide ion by chlorine dioxide has been studied by stopped-flow techniques at I = 1.0 M (NaClO4). The following two-term rate law was confirmed for the reaction : -d[ClO2]/dt = k(I)[CLO2][I-] + k(II)[ClO2][I-](2). The rate constants at 298 K and the activation parameters are k(I) = (1.87 +/- 0.02) x 10(3) M-1 s(-1), Delta H-I(double dagger) = 35.4 +/- 0.7 kJ/mol, Delta S-1(double dagger) = -63.5 +/- 2.3 J/(mol K), k(II) = (1.25 +/- 0.04) x 10(4) M-2 s(-1), Delta H-II(double dagger) = 36.7 +/- 1.3 kJ/mol, Delta S-II(double dagger) = -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.