X-Band (9.8 GHz) EPR measurements show that the free energy change controlling the electron transfer from the anion radical of perdeuterated [16]annulene to neutral [16]annulene (C16D16.-+ C16H16 reversible arrow C16H16.- + C16D16) is Delta G degrees = -0.65 kcal/mol. When supplied with vibrational frequencies from B3LYP/6-31 + G* calculations, the QUIVER program determines a Delta G degrees value of -0.142 kcal/mol. Thus, the experimental and theoretical values are in qualitative agreement. This result is the opposite of that obtained for the [8]annulene system, where the solution electron affinity of C8D8 proved to be slightly greater that that for C8H8. H-1 NMR (400 MHz) experiments reveal that the barrier to ring flattening is greater in the C16D16 system than in the C16H16 system. Coupled with the DFT prediction that the C16D16.- is nearly planar, this accounts for the equilibrium isotope effect observed in the electron transfer. The W-Band (94 GHz) EPR spectra, showing that the isotropic g-factor of the [16]annulene anion radical is not altered via perdeuteration, further support a nearly planar C16D16.-. The DFT calculations also predict that the dianion of [16]annulene is completely planar. However, deuteration of all but one of the hydrogens in C16H162- results in an upfield chemical shift of 0.086 ppm for the internal proton resonance at 153 K. This increase in ring current (pi-delocalization) is accounted for in terms of a C-C=C bond angle sigma-framework) relaxation, as shorter C-D bonds attenuate the internal steric interactions in C16D15H2-.