The reduction kinetics of hexaaquairon(III) was studied at different concentrations of perchloric acid at four single-crystal gold electrodes. The electron-transfer rate constants obtained at constant electrode potential increase in the following order: Au(210) < Au(110) < Au(100) < Au(111). The experimental transfer coefficients increase in the same order. An analysis of double-layer effects within the framework of the classical Frumkin approach was consistent with the transport of a reactant of charge 2+ through the double-layer region and gave a value of the apparent transfer coefficient (a) equal to 0.3 for all single-crystal electrodes studied. The alpha (a) value was rationalized by the distribution of charges within the reactant and product. The differences between double-layer effects on the reduction kinetics of hexaaquairon(III) and hexaamminecobalt(III) ions are discussed.