The kinetic isotope effects in the reactions of CH<sub>3</sub>Cl, <sup>13</sup>CH<sub>3</sub>Cl and CD<sub>3</sub>Cl with OH radicals and Cl atoms were studied in relative rate experiments at 298±2 K and 1013±10 mbar. The reactions were carried out in a smog chamber using long path FTIR detection and the spectroscopic data analyzed employing a non-linear least squares spectral fitting method using measured high-resolution infrared spectra as well as absorption cross sections from the HITRAN database. The reaction rates of <sup>13</sup>CH<sub>3</sub>Cl and CD<sub>3</sub>Cl with OH and Cl were determined relative to CH<sub>3</sub>Cl as: <i>k</i><sub>OH+CH3Cl</sub><i>k</i><sub>OH+CH3Cl</sub>/<i>k</i><sub>OH+13CH3Cl}</sub><i>k</i><sub>OH+13CH3Cl</sub>=1.059±0.008, <i>k</i><sub>OH+CH3Cl</sub><i>k</i><sub>OH+CH3Cl</sub>/<i>k</i><sub>OH+CD3Cl</sub><i>k</i><sub>OH+CD3Cl</sub>=3.9±0.4, <i>k</i><sub>Cl+CH3Cl</sub><i>k</i><sub>Cl+CH3Cl</sub>/<i>k</i><sub>Cl+13CH3Cl</sub><i>k</i><sub>Cl+13CH3Cl</sub> =1.070±0.010 and <i>k</i><sub>Cl+CH3Cl</sub><i>k</i><sub>Cl+CH3Cl</sub>/<i>k</i><sub>Cl+CD3Cl</sub><i>k</i><sub>Cl+CD3Cl</sub>=4.91±0.07. The uncertainties given are 2σ from the statistical analyses and do not include possible systematic errors. The unexpectedly large <sup>13</sup>C kinetic isotope effect in the OH reaction of CH<sub>3</sub>Cl has important implications for the global emission inventory of CH<sub>3</sub>Cl.