DFT calculations, including the effects of small curvature tunneling, have been performed on the reductive elimination of methane from hydridomethylbis(trimethylphosphine)platinum (1d). The calculations find that at 250 K tunneling results in an increase in the rate constant for reductive elimination by a factor of 4, a lowering of E. by 1.7 kcal/mol, and a decrease in A by a factor of nearly 10. Tunneling is also calculated to increase the primary HID kinetic isotope effect (KIE) from k(1d)/k(1f) = 2.26 to k(1d)/k(1f) = 4.12 1 and to result in a large secondary KIE of k(1d)/k(1e) = 1.35. In addition, tunneling is predicted to result in a violation of the rule of the geometric mean, so that the secondary KIE for reductive elimination of methane-d(1) from 1f is calculated to be k(1f)/k(1g) 1.06, which is much smaller than the secondary KIE of k(1d)/k(1e) = 1.35 for reductive elimination of methane from 1d. Comparison of the measured values of k(1d)/k(1e) and k(1f)/k(1g) is therefore proposed as an experimental test of the prediction that tunneling plays an important role in the reductive elimination of methane from 1d.