The title reaction was studied using long Oath length FTIR spectroscopy coupled to a smog chamber, In 1 Torr of N-2 diluent at 1-95 K a large kinetic isotope effect was observed, k(H)/k(D) = 6, suggesting that the reaction of Cl atoms with CH3I proceeds via H atom abstraction to give CH2I radicals and HCl with a rate constant of (9.0 +/- 1.8) x 10(-13) cm(3) molecule(-1) s(-1). In 700 Torr total pressure of N-2, O-2, and air diluents the reactivity of Cl toward CH3I was 50% greater than that at 1 Torr. The increase in reactivity is ascribed to the formation of the CH3I-Cl adduct. In 700 Torr of O-2 the majority (approximate to 98%) of the adduct decomposes back to reactants, and a small fraction (approximate to 2%) is lost via one or more processes that do not regenerate reactants. In 1 atm of air at 295 K the reaction of Cl atoms with CH3I produces less than 20% yield of CH3Cl. The potential role of the title reaction in atmospheric chemistry is discussed.