The solvation free energy curves for electron transfer between several types of ions in aqueous solution are studied by molecular dynamics computer simulations and by simple theoretical models. By using models of increasing complexity, contributions of different physical effects are evaluated. The theoretical models are for two ions at infinite separation in a dielectric continuum : the first is based on the Born solvation free energy, which assumes a linear response of the solvent and thus is basically Marcus theory, and the second is based on a nonlinear response model of ionic solvation by Hyun, Babu, and Ichiye (HBI), which includes the effects of dielectric saturation. Finally, molecular dynamics simulations of ions at infinite and finite separations describe the molecular nature of the solvent, with the latter including the influence of the solutes on each other. The focus here is on the orientational rather than electronic polarization, although the latter also will contribute. Previously, comparison of HBI and Born free energy curves showed that nonlinearities are most pronounced in electron transfer reactions involving a neutral to charged species or vice versa and become much less evident as the magnitude of charges of the solutes increases. Here, a comparison of results for ions at infinite separation from the molecular dynamics simulations and the HBI and Born models shows that dielectric saturation greatly reduced the activation energy Delta G(double dagger) mainly by shifting the free energy curves closer together (i.e., by reducing the polarization energy) but affected Delta G(double dagger) a lesser degree by the nonparabolic nature of the curve. Moreover, it shows that the contribution of the molecular structure of water such as density variations and hydrogen bonding was to shift the curves apart, resulting in a smaller increase in Delta G(double dagger). In addition, a comparison of molecular dynamics results for ions at infinite and finite separation shows that the effect of bringing the ions to a close separation was to reduce Delta G(double dagger) mainly by the reduction of the solvent reorganization at large distances, thus shifting the curves together. The direct influence of one solute on the polarization of the other was to increase the nonparabolic nature of the curve, which affects Delta G(double dagger) less.