The effect of the double layer on the reduction kinetics of [Co(NH3)(5)F](2+) cation was studied at four different single-crystal gold substrates. The electron-transfer rate constants increase in the order Au(210) < Au(110) < Au(100) < Au(111) at constant electrode potential. The value of the experimental transfer coefficient (alpha(ex)) also depends on the crystallographic orientation of the gold substrate and in general decreases with increasing concentration of the supporting electrolyte. The observed changes are smaller than those observed previously for the reduction of the [Co(NH3)(6)](3+) cation. Analysis of the double, layer effect for this system shows that the effective charge on the reactant is +1.6. The fact that the effective charge is lower than the formal charge of +2 is attributed to the presence of a dipole moment in the complex due to the anionic fluoride ligand. As a result, the complex is oriented at a positively charged interface with the fluoride anion in a region of higher positive potential, and the electrostatic double-layer effect is less than it would be for a reactant that is a point charge on the outer Helmholtz plane.