A model for outer-sphere bimolecular electron transfer reactions is presented which is based on a rate expression correct in the adiabatic, nonadiabatic, and intermediate dynamical regimes for an overdamped solvent coordinate. A complete discussion of the time-scale separations needed to define a bimolecular rate for these electron transfer reactions is given. Bimolecular reaction rates are predicted to display distinct solvent and electronic coupling dependencies, defined by a few experimentally variable parameters, which we now identify. Also, a significant intermediate dynamical regime is predicted to exist in which the rate is sensitive to both electronic coupling and nuclear relaxation. Evidence that both extreme dynamical regimes have been accessed experimentally is recalled, and strategies are presented for fully mapping out the dependence of rate on the electronic coupling and relaxation times. © 1988 American Institute of Physics.