Electrochemical rate constants k el and volumes of activation ΔV el ⧧ for self-exchange at an electrode of the aqueous couples Co(phen)3 3+/2+, Co(en)3 3+/2+, Fe(H2O)6 3+/2+, Co(diamsar)3+/2+, Co(diamsarH2)3+/2+, Co(sep)3+/2+, Co(ttcn)2 3+/2+, Fe(phen)3 3+/2+, Mo(CN)8 3-/4-, and Fe(CN)6 3-/4- have been measured by high-pressure AC voltammetry over the range 0.1−200 MPa at 25 °C; the respective values of ΔV el ⧧ are −9.1, −8.3, −5.5, −3.5, −3.8, −3.0, −2.8, −1.6, +7.3, and +11 cm3 mol-1. Although the theory of Marcus (Electrochim. Acta 1968, 13, 1005) suggests that ln k el should be linearly related to 1/2 ln k ex, where k ex is the rate constant of the corresponding homogeneous (bimolecular) self-exchange reaction, ln k el is often sensitive to the nature of the working electrode and the supporting electrolyte and is only weakly correlated with ln k ex, with slope ≈0.1. In contrast, ΔV el ⧧ = (0.50 ± 0.02)ΔV ex ⧧, in precise agreement with an extension of Marcus' theory, regardless of the nature of the electrode and the supporting electrolyte. This result implies that electron transfer in these couples occurs adiabatically on direct ion−ion and ion−electrode contact (i.e., within the outer Helmholtz plane), and also that ΔV el ⧧ values are predictable in the manner described elsewhere (Can. J. Chem. 1996, 74, 631) for ΔV ex ⧧. Conversely, where ΔV ex ⧧ cannot be measured for technical reasons (e.g., where paramagnetism of both reactants precludes NMR measurements of k ex), it can be reliably estimated as 2ΔV el ⧧.