The kinetics and mechanism of the reaction between IO and ClO radicals have been studied by the mass spectrometric discharge-flow method at T = 298 K and at total pressure around 1 Torr. The value of the overall rate constant k(1) = (1.1 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1) has been determined under pseudo-first-order conditions using an excess of ClO over IO radicals. The quantitative detection of the reaction products OClO, Cl, and ICl allowed for the determination of the branching ratios (0.55 +/- 0.03), (0.25 +/- 0.02), and (0.20 +/- 0.02) for the channels producing I + OClO, I + Cl + O-2, and ICI + O-2, respectively. Ln addition, the rate coefficient (3.2 +/- 0.5) x 10(-13) cm(3) molecule(-1) s(-1) has been measured at room temperature for the reaction I + Cl2O --> ICl + ClO. The kinetic data obtained allow for a redetermination of the heat of formation of IO : Delta H-f(IO) = (27.7 +/- 1.2) kcal mol(-1) (based on the current value of Delta H-f(OClO) = (22.6 +/- 1.0) kcal mol(-1)). The data obtained indicate that the reaction IO + ClO, which could be involved in stratospheric ozone depletion if iodine is present in the stratosphere, is much less important than assumed in previous model calculations.