The optical and thermal outer-sphere electron self-exchange reactions of the hexacyanoferrate(II/III) couple in concentrated solutions of the reactants in both H2O and D2O as the solvent have been studied by means of vis/near-IR and C-13-NMR spectroscopy, respectively. Spin-orbit splitting effects show up in the band shape of the optical transition and have to be considered also for the rate constant of the corresponding thermal reaction. The rate constant of the electron self-exchange reaction pathway, involving reactants and products only in the lowest electronic state, k(AA,calc)(H) = 5.1 X 10(4) M(-1) s(-1) (299 K, H2O), has been calculated from the band-shape parameters of the optical transition using a semiclassical model which is in a convincing agreement with the experimental value, k(AA,ex)(H) = 5.45 x 10(4) M(-1) s(-1) (299 K, H2O). A H2O/D2O kinetic isotope effect of 1.8 of both the observed and calculated rate constants has been found and interpreted in terms of the contribution of weakly hydrogen-bonded solvent molecules, present in the first solvational sphere of the reactants, to the reorganization free energies of both optical and thermal self-exchange processes.