This paper presents microelectrode voltammetry-derived heterogeneous electron transfer kinetic rates k(ET) for the redox couple [Co(bpy)(3)](2+/3+) in a series of solvents for which (tau(L)) longitudinal relaxation values are known (four polar monomeric solvents and four oligomeric polyether solvents, CH3O-(CH2CH2O)(n)-CH3 where n = 1, 2, 3, and 4) and one, a higher oligomer (n = 8, MPEG-400), for which tau(L) is estimated. tau(L) ranges from 0.2 to 38 ps. The results show that k(ET) varies inversely with tau(L), and according to other modes of analysis, as predicted for control of the energy barrier-crossing rate by the dynamics of solvent dipolar relaxation. Additionally, the observed k(ET) is proportional to the diffusion coefficient D-Co of [Co(bpy)(3)](2+), which is rationalized by the mutual connection of D-Co and k(ET) to the solvent viscosity. D-Co, k(ET), and viscosity were also measured as a function of electrolyte concentration in MPEG-400 which allowed extension of the overall solvent viscosity range. The [Co(bpy)(3)](2+/3+) rate constant in these media was also proportional to D-Co, indicating solvent dynamics control over a time scale range of ca. 500-fold, larger than any previously reported. Experiments at constant viscosity but varied electrolyte concentration demonstrated the absence of strong double layer or ion pairing influences on the reaction rate.