The greenhouse gas methane plays an important role in modeling global climate changes. The oxidation of CH4 by OH radical is the major sink of atmospheric methane in the troposphere. A theoretical study of the carbon-13 kinetic isotope effect (KIE) and its temperature dependence for the reaction OH + CH4 → H2O + CH3. The KIE values at various atmospherically significant temperature were determined by direct dynamics using variational transition state theory with multidimensional tunneling contributions. The potential energy surfaces were generated at a number of different levels of theory including both explicitly correlated wavefunctions and hybrid and doubly hybrid density functional theory methods. Factorization of the KIE into different contributions (potential energy, translation, rotation, vibration, and tunneling) provided insights into the origin of the KIE. This is an abstract of a paper presented at the 228th ACS National Meeting (Philadelphia, PA, 8/22-26/2004).