Electrotransport in oxides is an interesting phenomenon in two respects: (ii it gives insight into the microscopic mass and charge transport mechanisms in oxides, and (ii) due to the directed ionic fluxes, macroscopic phenomena such as demixing of an initially homogeneous multicomponent oxide can be studied. The electric field induces defect fluxes as a result of which ionic fluxes occur. Due to the different mobilities a concentration gradient will build up, and the initially homogeneous material becomes inhomogeneous. A formal analysis of the demixing phenomenon due to electrotransport is given for semiconducting oxide solutions and for doped oxygen ion conductors as well. Some experimental results and consequences for applications are discussed.