Methyl bromide (CH3Br) constitutes the largest source of bromine atoms to the stratosphere and plays an important role in stratospheric ozone depletion. However, the CH3Br atmospheric budget remains uncertain. We report the first measurements of stable carbon isotope ratios for atmospheric CH3Br and explore the constraints they provide for the atmospheric budget. The measured d13C of CH3Br ranged from 44.8% to 41.2% (mean value 43.1%, N = 7). This isotopic value is less negative than the anthropogenic source signature (53.2 to 47.5%), but is indistinguishable from the only quantified natural source signature (43.0%). Based on known source signatures and our atmospheric measurements, we predict that the mean kinetic isotope effect associated with sinks must be small (24%). We further estimate that a total phaseout of anthropogenic sources would likely change the isotopic composition of atmospheric CH3Br by 24%, and that this change would be difficult to detect with current measurement techniques. IsoPrime 穩(wěn)定同位素質(zhì)譜儀