<p>Chloromethane (CH<sub>3</sub>Cl) is the most important natural input of reactive chlorine to the stratosphere, contributing about 16 % to stratospheric ozone depletion. Due to the phase out of anthropogenic emissions of chlorofluorocarbons, CH<sub>3</sub>Cl will largely control future levels of stratospheric chlorine.</p> <p>The tropical rainforest is commonly assumed to be the strongest single CH<sub>3</sub>Cl source, contributing over half of the global annual emissions of about 4000 to 5000 Gg (1 Gg = 109 g). This source shows a characteristic carbon isotope fingerprint, making isotopic investigations a promising tool for improving its atmospheric budget. Applying carbon isotopes to better constrain the atmospheric budget of CH<sub>3</sub>Cl requires sound information on the kinetic isotope effects for the main sink processes e.g. the reaction with OH and Cl in the troposphere. We conducted photochemical CH<sub>3</sub>Cl degradation experiments in a 3500 L smog chamber to determine the carbon isotope fractionation (ε) for the reaction of CH<sub>3</sub>Cl with OH and Cl. For the reaction of CH<sub>3</sub>Cl with OH, we determined a ε of (−11.2 ± 0.8) ‰ (<i>n</i> = 3) and for the reaction with Cl we found a ε of (−10.2 ± 0.5) ‰ (<i>n</i> = 1) being five to six times smaller than previously reported. Our smaller isotope effects are strongly supported by the lack of any significant seasonal covariation in previously reported tropospheric δ<sup>13</sup>C(CH<sub>3</sub>Cl) values with the OH driven seasonal cycle in tropospheric mixing ratios.</p> <p> Applying these new fractionation factors to the global CH<sub>3</sub>Cl budget using a simple two hemispheric box model, we derive a tropical rainforest CH<sub>3</sub>Cl source of (670 ± 200) Gg a<sup>−1</sup>, which is considerably smaller than previous estimates. A revision of previous bottom up estimates, using above ground biomass instead of rainforest area, strongly supports this lower estimate. Finally, our results suggest a large unknown tropical CH<sub>3</sub>Cl source of (1230 ± 200) Gg a<sup>−1</sup>.</p>