The time evolution of the unmixing process in a semiconducting mixed oxide, (A,B)O, under an electric field was calculated by establishing the continuity equations in a phenomenological way and solving them numerically by means of a finite difference method. Temporal variations of composition and defect distribution, and boundary motions were obtained, and therefrom, the distribution of oxygen chemical potential inside the oxide was evaluated. It was found that, sluggish defect relaxation during the unmixing process might induce a gradient of the oxygen chemical potential in addition to the electrotransport-induced composition gradient. General aspects of an electrotransport-induced unmixing process are discussed.