In qualitative agreement with previous MP2 level calculations of the 12C/13C kinetic isotope effects (KIEs) for H-abstraction of CH4 by OH and Cl, higher level calculations yield KIEs at 300 K that differ from the corresponding experimental values, but in different directions. At QCISD(T)/6-311+G(2df,p), the theoretical KIE for CH4+Cl is 32‰ less than the measured value, while for CH4+OH, the theoretical KIE is greater than the measured value, but only by 5‰. Noticeable differences in atmospheric model simulation studies can occur if the measured KIEs are replaced by the analogous theoretical values. For example, the effect of carbon isotopic fractionation from the CH4 + Cl reaction on stratospheric δ13CH4 will be significantly lower than that obtained by Bergamaschi et al. . It is suggested, for the CH4+OH reaction and especially for the CH4+Cl reaction, that additional KIE measurements are needed.