approximation. Not only does instanton theory give accurate results but also direct insight into the reaction mechanism in terms of the optimal tunneling pathway, which indicates that the carbon atom dominates the tunneling. We thus propose that, since the tunneling effects in this system are so strong, even the small mass difference between C and 12 C could lead to measurable changes in the rate. By calculating the rates of thiophosgene isotopically substituted with C, we predict a kinetic isotope effect in the thermal rate of 1.4 at 300 K or in the microcanonical rates between 1.6 and 2.3 for the energy range shown in Fig. .