Cyclic voltammetry (CV) and X-ray diffraction (XRD) were used to investigate the anodic behavior of indium in a 3 M KOH solution on both rotating disc-platinum ring and static electrodes. Indium undergoes two processes in concentrated alkaline solution: activation and passivation. In the activation region, soluble, tri-valent indium is the main product, generated by the disproportionation reaction of In(OH)ad in the initial stage, and direct oxidation of indium during the latter stage. The soluble, tri-valent indium accumulates and precipitates as In(OH)3 on the electrode surface, inhibiting the oxidation of indium. In the resulting passivation region, the indium under the precipitated In(OH)3 is oxidized to In2O3, which is more stable than In(OH)3, and prevents further oxidation of the indium.