A binary oxide A(1-delta)O with cation disorder is simultaneously exposed to an oxygen potential and a temperature gradient. The side of the crystal which is exposed to the higher oxygen potential is always exposed to the higher temperature. Therefore a flux of cation vacancies from this side to the other and a corresponding flux of cations in the opposite direction is established. This leads to a displacement of the crystal surfaces. We have performed a linear stability analysis to investigate the morphological stability of the planar surfaces including the temperature dependence of the diffusion coefficients and the Soret effect which leads to the following results : Compared with the corresponding isothermal case there is a reversal in the stability behaviour caused by the heat of transport Q for Q<0 : The reducing side will become stable, the oxidizing one unstable. First experiments which we carried out on Co1-deltaO show a significant change in the stability behaviour at the reducing side.