Data for the title reaction have been fitted using an RRKM/master equation approach. Energy transfer was modeled using an exponential decay with downward step sizes, E d , as a fitting parameter. The low temperature (200 < T (K) < 300) combination of CH 3 with Cl atoms in He can be accommodated with E d (cm −1 ) = 400. Higher temperature (1600 < T (K) < 2100) decomposition in Ar required E d (T ) (cm −1 ) = 694(T /300) 0.46 . Previous analysis of the analogous system CH 4 = CH 3 + H required E d (T ) (cm −1 ) = 100(T /300) for He and E d (T ) (cm −1 ) = 150(T /300) for Ar. Understanding of the magnitudes and temperature dependence of E d remains the greatest detriment to quantitative calculation, extrapolation, and prediction of unimolecular rate constants.