The rate constant (k(q)) for the quenching of *Ru(bpy)(3)(2+) by methylviologen (MV(2+)) has been determined as a function of the mole fraction of CH3CN (X(AN)) in aqueous mixtures; k(q) goes through a minimum at X(AN) similar to 0.4, which is suggested to occur because of the dynamic solvent effect for the electron-transfer reaction in the mixed solvents, whereby the relaxation dynamics of the solvent is a strong function of its composition. The cage escape yield (eta(ce)) for the release of the redox products into bulk solution has been determined as a function of ionic strength, temperature, and X(AN); the apparent activation energy for the back electron transfer (E(bt)) between the geminate redox pair, Ru(bpy)(3)(3+) and MV(.+), within the solvent cage produced upon quenching is obtained from the data through the application of the simple cage model. E(bt) is independent of ionic strength in aqueous solutions, but is strongly dependent on solvent composition in mixed solvents. However, E(bt) decreases smoothly from 9.6 kJ mol(-1) in aqueous solution to -5.5 kJ mol(-1) in CH3CN-rich solvents as X(AN) is increased, requiring that a modification of the simple model be made. The concept of a kinetically important reorientation of the geminate redox pair is introduced to account for the experimental observations; the simple solvent cage model can be viewed as a limiting case in the modified model.