Gallium arsenide was anodically oxidized in a mixture of ethylene glycol and tartaric acid as an electrolyte. The numbers of Ga and As atoms in the anodic oxide films and in the used electrolytes were measured by radioactivation analysis. During the anodic oxidation, GaAs dissolved into the electrolyte. The numbers of Ga and As atoms that dissolved into the electrolytes was proportional to the anodic voltage, and the number of Ga atoms in the electrolyte was about five times more than that of As atoms. The composition of the anodic oxide films varied with depth. However, the atomic profiles measured by Auger electron spectroscopy displayed As atoms much less than Ga atoms throughout the anodic oxide films, because Ga oxides were lost from the anodic oxide films into the vacuum during the Auger electron spectroscopy, accompanying sputtered thin film removal.