Ozone reactions with XO2- (X = Cl or Br) are studied by stopped-flow spectroscopy under pseudo-first-order conditions with excess XO2-. The O-3/XO2- reactions are first-order in [O-3] and [XO2-] with rate constants k(1)(Cl) = 8.2(4) x 10(6) M-1 s(-1) and k(1)(Br) = 8.9(3) x 10(4) M-1 S-1 at 25.0 degreesC and mu = 1.0 M. The proposed rate-determining step is an electron transfer from XO2- to O-3 to form XO2 and O-3(-). Subsequent rapid reactions of O-3(-) with general acids produce O-2 and OH. The OH radical reacts rapidly with XO2- to form a second XO2 and OH-. In the O-3/ClO2- reaction, ClO2 and ClO3-are the final products due to competition between the OH/ClO2-reaction to form ClO2 and the OH/ClO2 reaction to form ClO3-. Unlike ClO2, BrO2 is not a stable product due to its rapid disproportionation to form BrO2- and BrO3-. However, kinetic spectra show that small but observable concentrations of BrO2 form within the dead time of the stopped-flow instrument, Bromine dioxide is a transitory intermediate, and its observed rate of decay is equal to half the rate of the O-3/BrO2-reaction, Ion chromatographic analysis shows that O-3 and Bro(2)(-) react in a 1/1 ratio to form BrO3- as the final product. Variation of k(1)(x) values with temperature gives DeltaH(Cl)(double dagger), = 29(2) kJ mol(-1), DeltaS(Cl)(double dagger) = -14.6(7) J mol(-1) K-1, DeltaH(Br)(double dagger) = 54.9(8) kJ mol(-1), and DeltaS(Br)(double dagger) = 34(3) J mol(-1) K-1. The positive DeltaS(Br)(double dagger) value is attributed to the loss of coordinated H2O from BrO2- upon formation of an [O-3/BrO2-](double dagger) activated complex.