<p>The <sup>13</sup>C isotopic ratio of methane, δ<sup>13</sup>C of CH<sub>4</sub>, provides additional constraints on the CH<sub>4</sub> budget to complement the constraints from CH<sub>4</sub> observations. The interpretation of δ<sup>13</sup>C observations is complicated, however, by uncertainties in the methane sink. The reaction of CH<sub>4</sub> with Cl is highly fractionating, increasing the relative abundance of <sup>13</sup>CH<sub>4</sub>, but there is currently no consensus on the strength of the tropospheric Cl sink. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the δ<sup>13</sup>C of CH<sub>4</sub> to the diversity of Cl output from chemical transport models. We find that δ<sup>13</sup>C is highly sensitive to both the amount and geographic distribution of Cl. Simulatlons with Cl providing 0.28 % or 0.66 % of the total CH<sub>4</sub> loss bracket the δ<sup>13</sup>C observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total CH<sub>4</sub> loss and has a small impact on total CH<sub>4</sub>, it provides a strong lever on δ<sup>13</sup>C. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of δ<sup>13</sup>C. The large effect of Cl on δ<sup>13</sup>C compared to total CH<sub>4</sub> broadens the range of CH<sub>4</sub> source mixtures that can be reconciled with δ<sup>13</sup>C observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH<sub>4</sub> sources from δ<sup>13</sup>C observations.<p>