The effect of vibrations in the inner-sphere of reacting molecules on the outer-sphere electron-transfer is investigated. We use a classical model of the solvent and a quantum two-state electron subsystem. In addition, we include a quantum mechanical treatment of the vibrational modes of the inner-sphere. A general derivation of the rate constant is given. Using a continuous model for the solvent a usable form of the rate constant is obtained. The various effects of the vibrations on the rate constant are investigated. Considerable effects are noticed for very exothermic and endothermic reactions. Some relevant experimental evidence is discussed.