Carbonyl-C-13, O-17, and C6D5 isotope effects (IEs) on tile electron transfer equilibrium between benzophenone and benzophenone radical anion were determined by means of fourier transfer ion cyclotron resonance (FT-ICR) measurements and the ab initio MO calculations (UHF/6-31G*). The experimental and theoretical IEs are in excellent agreement with each other (K12/K13 = 1.03 (50 degrees C, ICR), 1.0:26 (50 degrees C, MO) and KD5/KH5 = 1.35 (50 degrees C, ICR), 1.325 (50 degrees C, MO)). The carbonyl-C-13, O-17, ring C-13(12), and D-10 Ifs were calculated as 1.044, 1.025, 1.206, and 2.812, respectively, at -75 degrees C. The carbonyl-C-13 and O-17 IEs disagree with the reported IEs in liquid NH3 (R(12/)K(13) = 2.0, K-16/K-17 = 0.26, at -75 degrees C, Stevenson et al. J. Phys, Chem. 1988, 92, 3687) and are close to unity as generally expected. The present results do not support the explanation in terms of an ion association effect in the liquid phase.