The effect of the double-layer on the reduction of [Co(NH3)(6)](3+) cation has been studied at four different single-crystal gold substrates, namely, Au(111), Au(100), Au(110), and Au(210), and at various concentrations of perchloric acid. The values of the experimental transfer coefficient (alpha(ex)) are much larger than 1.0 at all gold surfaces and decrease with increasing concentration of the supporting electrolyte. The value of alpha(ex) also depends on the crystallographic orientation of the gold substrate and increases in the following order: Au(111) < Au(100) < Au(110) < Au(210), consistent with a decreasing reduction rate from Au(111) to Au(210) at constant electrode potential. The experimental transfer coefficients are rationalized on the basis of the classical theory of the double-layer with consideration of reactants and products in estimation of the diffuse-layer potential. The charge on the reactant transported through the double-layer is 2+. Corrected Tafel plots yield an apparent transfer coefficient alpha(a) equal to 1.1 +/- 0.1 for all analyzed substrates. Corrected rate constants For double-layer effects indicate that the reduction of [Co(NH3)(6)](3+) ion depends on the crystallographic orientation of the gold substrate only through the differences in the potential of zero charge. This behavior confirms char the role of the metal is that expected for an adiabatic outer-sphere electron-transfer process.