The enthalpy of formation of the free ions generated by the electron transfer between benzophenone in the lower triplet state and 1,4-diazabicyclo[2,2,2]octane in acetonitrile has been measured using transient thermal phase grating. This enthalpy is smaller by more than 0.14±0.08 eV than the enthalpy of formation of the geminate ion pair obtained from a previous ps thermal phase grating investigation. Therefore, the dissociation of a geminate ion pair into free ions in endothermic. However, separation is exergonic if translational entropy in taken into account. A small and positive value of C, the correction term in the Rehm—Weller equation, is obtained if this term is considered as a free energy. The principles of the transient grating technique as a tool for investigating photoinduced processes in solution are briefly reviewed.