The atmospheric reaction between I-IS and chlorine radicals was studied theoretically, using ab initio methods. We investigated three reaction possibilities: HSCl formation, and the production of HCl and sulfur, singlet and triplet. The channels that lead to HSCl and to HCl and S(P-3) are spontaneous and exothermic at 298.15 K, and the channel for HCl and S(D-1) formation is nonspontaneous and endothermic. In a microcanonical kinetics calculation, the rate constant we obtained for HSCl formation was 2.4 x 10(-11) cm(3) molecule(-1) s(-1) at a pressure of 1 arm, and 2.4 x 10(-13) cm(3) molecule(-1) s(-1) at a pressure of 0.01 arm. The rate constant for HCl and triplet sulfur production was not significantly pressure-dependent, and its value was 9.0 x 10(-11) cm(3) molecule(-1) s(-1). In the troposphere, the proportion of reaction products predicted was 79% for HCl and S(P-3), and 21% for the formation of HSCl. At lower pressures, the proportion of HSCl decreased, and at a pressure of 0.01 atm it was predicted to be practically null. The rate constant for the total reaction was determined to be 1.1 x 10(-10) cm(3) molecule(-1) s(-1) at 1 atm. At a pressure of 0.01 atm, this value decreased to 9.0 x 10(-11) cm(3) molecule(-1) s(-1), which is within the limit error of the only experimental result known in the literature.