The volume of activation Delta V-el(double dagger) for the Co(bpy)(3)(3+/2+) electrode reaction in aqueous NaCl (0.2 mol L-1) is -8.6 +/- 0.4 cm(3) mol(-1) at 25.0 degrees C, as expected on theoretical grounds and by analogy with Co(en)(3)(3+/2+) and Co(phen)(3)(3+/2+), and neither the rate constant k(el) at various pressures nor Delta V-el(double dagger) correlate with the corresponding mean diffusion coefficients D for the couple and the diffusional activation volume Delta V-diff(double dagger), respectively. In organic solvents, however, Delta V-el(double dagger) is strongly positive (9.1 +/- 0.3, 10.2 +/- 0.7, and 12.2 +/- 0.9 cm(3) mol(-1) for CH3CN, acetone, and propylene carbonate, respectively, with 0.2 mol L-1 [(C4H9)(4)N]ClO4 at 25 degrees C) and correlates with Delta V-diff(double dagger), while k(el) correlates with D. These results support the proposition of Murray et al. (J. Am. Chem. Sec. 1996, 118, 1743; 1997, 119, 10249) that solvent dynamics control the rate of the Co(bpy)(3)(3+/2+) electrode reaction in organic solvents. In aqueous solution at near-ambient temperatures, solvent dynamical influences would not be revealed by pressure effects, but in any event the aqueous CO(bpy)(3)(3+/2+) electrode reaction appears to be mechanistically different from the nonaqueous cases. For the reduction of Co(bpy)(3)(3+) With Co(sep)(2+) in homogeneous aqueous solution, the rate constant is lower, and the volume of activation more negative, than can be accommodated by extended Marcus theory, suggesting nonadiabatic behavior. These observations are consistent with the view that, although the self-exchange and electrode reactions are generally adiabatic, cross reactions involving Co-III/II couples (and presumably others) become increasingly nonadiabatic as the driving potential is increased.