Electrochemical rate constants k(el)(double dagger) and volumes of activation Delta V-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(diamsarH(2))(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 degrees C; the respective values of Delta Vel(double dagger) are -9.1, -8.3, -5.5, -3.5, -3.8, -3.0, -2.8, -1.6, +7.3, and +11 cm(3) mol(-1). Although the theory of Marcus (Electrochim. Acta 1968, 13, 1005) suggests that In k(el) should be linearly related to 1/2 ln k(ex), where k(ex) is the rate constant ofthe 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 approximate to 0.1. In contrast, Delta V-el(double dagger) = (0.50 +/- 0.02)Delta V-ex(double dagger), 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 Delta V-el(double dagger) values are predictable in the manner described elsewhere (Can. J. Chem. 1996, 74, 631) for Delta V-ex(double dagger). Conversely, where Delta V-ex(double dagger) cannot be measured for technical reasons (e.g., where paramagnetismof both reactants precludes NMR measurements of k(ex)), it can be reliably estimated as 2 Delta V-el(double dagger).