The isotope effects (IEs) on the one- and two-electron reductions of cyclooctatetraene (COT) have been studied by using the semiempirical PM3 method. The one-electron reduction is calculated within the ROHF formalism to yield a nonplanar radical anion. The hydrogen compound is calculated to be preferentially reduced rather than the deuterium compound, as observed and calculated earlier for aromatic systems. This is in contrast with the experimental data for the one-electron reduction of COT but in line with the experimental IE on the addition of the second electron. Ion-pair formation and population of higher vibrational levels in the anions are discussed as possible causes for the deviation between theory and experiment, and the first of these is concluded to be the most important.