The sulfite-chlorine dioxide reaction was studied by stopped-flow method at I = 0.5 M and at 25.0 +/- 0.1 degrees C in a slightly acidic medium. The stoichiometry was found to be 2SO(3)(2-) + 2(.)ClO(2) + H2O -> 2SO(4)(2-) + Cl- + ClO3- + 2H(+) in (ClO2)-Cl-. excess and 6SO(3)(2-) + 2(.)ClO(2) -> S2O62- + 4SO(4)(2-) + 2Cl(-) in total sulfite excess ([S(IV)] = [H2SO3] + [HSO3-] + [SO32-]). A nine-step model with four fitted kinetic parameters is suggested in which the proposed adduct (SO3ClO22-)-S-. plays a significant role. The pH-dependence of the kinetic traces indicates that SO32- reacts much faster with (ClO2)-Cl-. than HSO3- does.