Thermal chlorine atoms react with gaseous CH3I and CH3Br at 295 K to produce CH3Cl with yields of 8.6% and 0.6%, respectively. The studies utilized radioactive 38Cl formed by the 37Cl (n,&ggr;) 38Cl nuclear reaction in gaseous CClF3 at pressures from 760 to 4,000 torr. The initially energetic 38Cl atoms are thermalized by multiple inelastic collisions with CClF3 prior to reaction with the methyl halide. The reaction rate constant for thermal chlorine attack on CH3Br to form CH3Cl at 295K and 1--5 atmospheres pressure is (2.0¿0.5)¿10-15 cm3 molecule-1 s-1. The substitution of Cl for Br/I in a methyl halide molecule cannot be a significant source of atmospheric CH3Cl because of its abundance from other sources. However, the replacement of I by Cl in other iodohalocarbons might in some cases provide a marker for the presence of atomic chlorine. ¿ 1997 American Geophysical Union