A two-dimensional model is used to evaluate the impact of stratospheric chlorine on δ13C of CH4 at the surface and to analyze sensitivity to different chemical and transport processes. It is found that the impact of Cl depends on the relative strength of the lower and upper branches of the stratospheric circulation, which emphasize regions of differing rates of methane fractionation. It is estimated that the increase in concentrations of Cl associated with human activity has contributed at most 0.54? to the 2? increase of δ13C observed over the past century. It follows that growth in the relative importance of heavier sources of CH4 (associated, for example, with fossil fuels and biomass burning) has been largely responsible for the trend.