It has recently been reported [Mroz et al., 1989> that the abundance of mass-20 isotopes of methane (CH4), which correspond to either of the highly deuterated forms 12CD4 or 13CHD3, is some 500 times that expected based on the statistical combination of H, D, 12C, and 13C and observed CH4 amounts. These authors then suggested that it is possible that the enhanced concentration of these species is due to their longer lifetimes than that of CH, itself because of their slower rate of loss by reaction with the hydroxyl radical (OH). We have tested this hypothesis with a two-dimensional atmospheric chemical-dynamical model and found that no large enhancements of 13CHD3 and 12CH4 can result in this way; in the troposphere enhancement factors of between 4 and 5 and between 7 and 8, respectively, were found, with enhancements becoming only slightly larger (14 and 25, respectively) in the stratosphere. The factor of 500 enhancement reported by Mroz et al. must have other origins.