A series of experiments were carried out to determine the clumped ( 13 CH 3 D) methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the major sink reactions for atmospheric methane. Experiments were performed in a 100 L quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) isotopologue ratios using tunable infrared laser direct absorption spectroscopy. Measured kinetic isotope effects for singly substituted species were consistent with previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. This is consistent with model calculations performed using quantum chemistry and transition state theory. The OH and Cl reactions enrich the residual methane in the clumped isotopologue in open system reactions. In a closed system, however, this effect is overtaken by the large D/H isotope effect, which causes the residual methane to become anti-clumped relative to the initial methane. Based on these results, we demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric methane, will only have a minor (∼0.3‰) impact on the clumped isotope signature (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on methane sink strengths. [ABSTRACT FROM AUTHOR]