A generalized Zusman equation and its formal solution for electron transfer reactions in a non-Debye medium with frequency-dependent friction are presented. The derivation is based on the spin-boson model, representing a two-level system for a donor and an acceptor coupled to a non-Debye polar solvent bath. An analytical expression for the electron transfer rate constant will be derived using the Green’s function method. Because of the time retardation in such a non-Markovian process, the initial electron-transfer reaction is influenced more by the higher frequency components in the solvent relaxation, whereas the long-time behavior is influenced more by the lower frequency components. Electron transfer processes in such a medium are therefore often nonexponential.