A study of the energetics of electron transfer (e.t.) quenching of 9-cyanoanthracene fluorescence by aromatic (pi) and aliphatic (n) donors in n-hexane using picosecond transient thermal phase grating spectroscopy is reported. The results show that the enthalpy of exciplex formation is well correlated with the redox potentials of the partners and independent of the nature of the donor, as long as exciton interaction is small. The main difference between exciplexes with donors of the two classes is the larger ground-state repulsion energy with n-donors. However, if the adiabatic ionization potentials are considered, the enthalpy of exciplex formation with n-donors is larger than expected by about 0.4 eV. These two contradictory results reflect an inconsistency between the values for adiabatic ionization potentials and oxidation potentials of n-donors. The occurrence of two distinct Rehm-Weller plots for e.t. quenching with pi- and n-donors shifted by about 0.8 eV on the horizontal axis seems to originate to some extent from the use of adiabatic ionization potentials.