Isosbestic points in the transient absorption spectrum for an intermolecular photoinduced electron-transfer system (PET) (bis-(diphenylamino)diphenylbutadiene pills C-70 in dilute solutions) are observed. An analytical treatment of these points indicate that they are a measure of the quantum efficiency for the PET process. Variation of parameters that affect PET quantum efficiency, such as solvent polarity and photoinitiator lifetime, result in predicted frequency shifts of the isosbestic point. Comparison to established methods of determination of quantum efficiency indicates that the method is quantitatively reliable as well as qualitatively useful. Application of the methodology to other systems is discussed.