The equilibrium isotope effects (EIE) for the one-electron transfer between pyrene and seven regioselectively deuteriated pyrene isotopic isomers in dimethylformamide with 0.1 M tetrabutylammonium hexafluorophosphate were measured electrochemically. These data correlate linearly with the free energies (Delta G(o)) obtained in tetrahydrofuran using electron paramagnetic resonance (EPR) techniques. However, the slope of the resulting line is not unity, and it indicates that the EIE in the DMF system is only two-thirds of that in the THF system. PM3 calculated Delta G(o,)s, which would correspond to the gas phase electron transfers, also correlate linearly with both sets of experimental data, but the predicted magnitudes of the EIE’s are smaller than those observed experimentally by either technique. The nonunity slopes probably reflect slight differences in ion solvation and/or ion association parameters between the anion radicals of the isotopic isomers. No general relationship between the EIE and the charge on the hydrogen/deuterium substituted carbon atom was found.