B3LYP/6-31+G* density functional calculations have been performed in order to understand why large, equilibrium isotope effects have been observed in the reduction of benzene to the corresponding radical anion but not in the reduction of cyclooctatetraene (GOT) to COT.-. The calculations reproduce these experimental trends. However, the calculations also find that reduction of planar GOT, like that of benzene, has a substantial, equilibrium isotope effect. Therefore, the small isotope effect, both calculated and observed, for reduction of COT is due to an inverse isotope effect on the planarization of the tub-shaped ring in GOT. The isotope effect computed for forming planar (D-8h) triplet COT/COT-d(8) at 173 K is 0.38. This is the predicted isotope effect on the adiabatic singlet-triplet splitting in GOT. The isotope effect computed for forming planar (D-4h) Singlet COT/COT-d(8) at the same temperature is 0.41. This is the predicted isotope effect on ring inversion. The reason why planarization of COT has an inverse isotope effect is discussed, and an experimental test of the prediction of an inverse isotope effect on ring inversion of COT is proposed.