Self-assembled monolayers cannot be used to readily prepare insulating layers that are a few nm thick and are pinhole-free onto electrodes. Here, atomic layer deposition (ALD) is used to prepare aluminum oxide (Al2O3) insulating layers, whose thickness is controllable and which are pinhole-free toward electroactive species such as Ru(NH3)(6)(3+) and ferrocenemethanol, on indium tin oxide electrodes. The controllability of the uniform thickness of the Al2O3 layers enables us to investigate the dependence of the total capacitance between an electrode and an electrolyte solution on the thickness of the ALD layer over a wide range of thicknesses. When the thickness is greater than ca. 2 nm, the total capacitance is dominated by only the ALD layer capacitance. Consequently, the reciprocal of the total capacitance is linearly proportional to the ALD layer thickness. From this finding, a schematic diagram for the potential profile across the ALD layer and in the electrolyte solution is proposed. (C) 2013 Elsevier B.V. All rights reserved.