The ¹³CH₃D/¹²CH₄kinetic isotope effect, α¹³CH₃D, of CH₄ + Cl is determined for the first time, using the relative rate technique and Fourier transform infrared (FTIR) spectroscopy. α¹³CH₃D is found to be 1.60 ± 0.04. In addition, a quantum chemistry/transition state theory model with tunneling correction is constructed and the primary cause for α¹³CH₃D is found to be the substantially reduced reactivity of the D atom, which, in turn, can be explained by a significant increase in the reaction barrier due to changes in the vibrational zero point energy and to a lesser extent tunneling.