The theory of outer-sphere electron tranfer processes has been developed by many scientists since the first publication by Libby in 1952 referring to the role of the solvent. After the establishment of Marcus Theory, the electron transfer reactions have been analysed on the basis of activation free energy, provided that the inner-sphere and outer-sphere changes are concerted. However, this approach was proved to be rather inaccurate because of the uncertainties in the estimation of the inner-sphere contribution to the activation free energy. A recent development of the analyses based on the volume profiles of reactions, however, proved that the more accurate interpretation of the outer-sphere electron transfer processes is possible as the uncertainty related to the inner-sphere contribution diminishes. Theoretical treatment and the application of the "volume analysis" to various outer-sphere reactions are summarized in this review, together with an application of the volume analysis to the "Gated" electron transfer processes.
