The time evolution of the demixing process in a transition metal oxide solid solution, (A.B)(1-delta)O, under a stationary oxygen potential gradient is calculated by solving the governing continuity equations numerically with a finite difference technique. The transient solution of the chemical diffusion process of a monoxide is also computed. From the time variations of the demixing profile and the spatial distribution of the chemical potential of oxygen in the oxide, it is found that two transient processes are involved in demixing leading to the steady state: (1) the establishment of a gradient in the chemical potential of oxygen across the system oxide; and (2) the redistribution of the cations leading to demixing.