The (CH3D)-C-13/(CH4)-C-12 kinetic isotope effect, alpha((CH3D)-C-13), of CH4 + Cl is determined for the first time, using the relative rate technique and Fourier transform infrared (FTIR) spectroscopy. alpha((CH3D)-C-13) 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 alpha((CH3D)-C-13) 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. (C) 2014 Elsevier B.V. All rights reserved.