The marked dependence of the rate constant k2 for the Co(phen)33+/2+ self-exchange reaction in water on the nature and concentration of the anion X− can be accommodated in terms of the full Debye-Hückel equation taking ion size parameter à as adjustable (520 pm for X = Cl, 360 pm for X = NO3). A small additional contribution to the exchange rate, most evident at low electrolyte concentrations, appears to come from a heterogeneous pathway. At infinite dilution, k2 is estimated to be 0.35 L mol−1 s−1 at 25 °C; cf. 6.7 and 10.7 L mol−1 s−1 for X = Cl and NO3, respectively, at ionic strength I = 0.107 mol L−1. By contrast, the corresponding volume of activation ΔV* shows only a small dependence on I (−20.1 ± 0.4 cm3 mol−1 as I → 0; cf. −17.6 ± 0.7 and −16.0 ± 0.7 cm3 mol−1 for X = Cl and NO3, respectively at I = 0.107 mol L−1), and this is accurately calculable from Debye-Hückel-based theory (Inorg. Chem. 1990, 29, 5017). Within the limits of detection, Co(phen)32+ is entirely high-spin in aqueous solution (μeff = 4.79 μB with θ = 20 K, 13–85 °C). For Co(phen)33+/2+, ΔV* is much more negative (by some 15 cm3 mol−1) than adiabatic electron-transfer theory can accommodate, and this appears to be typical of low-spin/high-spin CoIII/II self-exchange reactions such as Co(en)33+/2+, except for Co(sep)3+/2+, for which ΔV* conforms well to predictions for adiabatic electron transfer. The behavior of Co(phen)33+/2+ and Co(en)33+/2+ is attributed to either adiabatic electron transfer from a low-spin isomer of the CoII complex in highly unfavorable equilibrium with the high-spin form or, more likely, nonadiabatic electron transfer involving the CoII and CoIII ground states.