A well-defined wave for the oxidation of hydroxide ions was recorded with gold microdisc electrodes either in sodium sulphate or sodium perchlorate solutions made basic with sodium hydroxide. The wave appeared at about 1.3 V versus SCE, just before the background discharge. The plot of the limiting current corresponding to this wave against the radius of a series of microdisc electrodes gave a straight line which indicated that the process is diffusion controlled. Similarly, the limiting current was found to depend linearly on pH over the range 9-12.5. To our knowledge, the results presented in this article provide the first experimental evidence for the diffusion-controlled electrochemical oxidation of hydroxide ions in aqueous solutions.