The kinetics and mechanism of reaction 1 between IO and BrO radicals have been studied by the mass spectrometric discharge flow method at 298 K. The value of the overall rate constant kappa(1) = (8.5 +/- 1.5) x 10(-11) cm(3) molecule(-1) s(-1) has been determined from the simultaneous monitoring of BrO and IO. The branching ratios have been measured for the various channels of reaction 1 in different series of experiments. The branching ratio for the major channel, producing Br and OIO, has been found in the range 0.65-1.0. The upper limits of the branching ratios for the channels producing I + Br + O-2, I + OBrO, and IBr + O-2 are 0.3, 0.2, and 0.05, respectively, whereas an upper Limit of 0.3 has been found for the sum of the branching ratios for the I-atom-producing channels. Rate coefficients for side reactions have been also redetermined under the conditions of the present study: BrO + BrO --> products, I + O-3 --> IO + O-2, NO + O-3 --> NO2 + O-2, Br + IO --> I + BrO, BrO + I-2 --> products, and BrO + IBr --> products. The kinetic data obtained for the IO + BrO reaction allow one to derive an upper Limit for the enthalpy of formation of OIO: Delta H degrees(f,298),298 (OIO) less than or equal to 32.2 kcal mol(-1). The atmospheric implications of the present data are briefly discussed.