In order to estimate carbon and hydrogen kinetic isotope effects for the reaction Cl+CH4 at atmospheric temperatures, measured values of the rates of Cl reacting with 13CH4, 12CH3D, 12CH2D2, 12CHD3, and 12CD4 relative to those of Cl+12CH4 were fit using a model that accounts for quantum mechanical tunneling. The model also includes vibrational enhancement of the reaction rate by excitation of the bending and stretching modes of CH4. These fits allow extrapolation of measured values to temperatures outside the range in which they were measured. In addition, the kinetic isotope effect for Cl+14CH4 was predicted using this model based on the results for Cl+13CH4. The model results for Cl+14CH4, Cl+13CH4, and Cl+12CH3D were parameterized for use in atmospheric models. ¿ 2001 American Geophysical Union