There are many important processes in corrosion for which the diffusion-limited current density of oxygen reduction, i(L), plays a dominant role in terms of kinetic control. The conventionally accepted value of i(L) (for the four-electron reduction mechanism) which can be found in many corrosion textbooks is in the range of 50-100 mA cm(-2), but the origins of this range of values are a bit mysterious. Previous research in our group aimed at ascertaining i(L) (under stagnant conditions) on a planar Cu electrode and a Cu microelectrode array in a naturally aerated 0.1 M Na2SO4 electrolyte found that i(L) was in the range of 20-30 mA cm(-2). In situ scanning tunneling microscopy was used to characterize the Cu surface at relevant potentials. Rotating disk electrode studies were used to measure i(L) in a naturally aerated 0.1 M Na2SO4 electrolyte as a function of pH for both Pt and Cu electrodes. By comparing results for Pt and Cu we conclude that oxygen reduction occurs on a Cu surface via the four-electron mechanism. The oxygen diffusion-limited current density was found to be independent of pH (in the range 1.5-14). Finally, we conclude that our previous determination of i(L) in a stagnant electrolyte reflects an accurate range of values of the oxygen diffusion-limited current density.