The steady state demixing of an initially homogeneous oxide solid solution (A, B)O in an oxygen potential field is studied theoretically and experimentally. In case that DA > DB ≫ D0, the crystal is shifted with respect to the oxide lattice system toward the higher oxygen potential and is enriched in A at the side of the higher oxygen potential, while the transport of oxygen in the crystal is negligible. A numerical solution of the transport problem is presented, and the predicted effect is verified experimentally