The reduction Of ClO2 to ClO2- by aqueous iron(II) in 0.5 M HClO4 proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M-1 s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO22+ that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M-1 s(-1)) is controlled by ClO2 substitution of a coordinated water to give an inner-sphere complex between ClO2 and Fe(II) that very rapidly transfers an electron to give (Fe-III(ClO2-)(H2O)(5)(2+)) (Is). The composite activation parameters for the ClO2/Fe(aq)(2+) reaction (innersphere + outer-sphere) are the following: DeltaH(r)(double dagger) = 40kJ mol(-1); DeltaS(r)(double dagger) = 1.7 J mol(-1) K-1. The (FeClO22+)-Cl-III inner-sphere complex dissociates to give Fe(aq)(3+) and ClO2- (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)(double dagger) = 76 kJ mol(-1); DeltaS(d)(double dagger) = 32 J K-1 mol(-1). The reaction of Fe(aq)(2+) with ClO2- is first order in each species with a second-order rate constant of k(ClO2)(-) = 2.0 x 10(3) M-1 s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO2 in H2SO4 medium ([H+] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H2SO4 are DeltaH(cl(lll))(double dagger) = 41 kJ mol(-1) and DeltaS(Cl(III))(double dagger) = 48 J mol(-1) K-1.