The kinetics of the reactions of iodine atoms with OClO and ClO and of chlorine atoms with I-2 and IO have been studied using the discharge-flow mass spectrometric method at a total pressure of about 1 Torr. The rate constant obtained for the reaction I + OClO --> IO + ClO (2) in the temperature range (288-353) K is k(2) = (9.7 +/- 3.0) x 10(-12) exp(-(1190 +/- 200)/T) cm(3) molecule(-1) s(-1). IO and ClO radicals have been detected as reaction products, and a branching ratio of unity has been found for the channel forming these two radicals. For the reaction I + ClO --> products (3), only upper limits of the rate constant have been determined : k(3) less than or equal to 3.7 x 10(-15) and k(3) less than or equal to 1.7 x 10(-14) cm(3) molecule(-1) s(-1) at T = 297 and 353 K, respectively. For the reaction between Cl atoms and IO radicals, Cl + IO --> I + ClO (4), the rate constant has been observed to be independent of temperature in the range 290-355 K : k(4) = (4.4 +/- 1.0) x 10(-11) cm(3) molecule(-1) s(-1). Finally, the rate constant obtained for the reaction Cl + I-2 --> I + ICl (5) is k(5) = (2.1 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) in the temperature range 296-365 K, From these kinetic data, upper and lower limits for the enthalpy of IO formation at T = 298 K could be derived : 25.8 kcal mol(-1) less than or equal to Delta H-f(IO) less than or equal to 27.5 kcal mol(-1).