Steady-state kinetic demixing in an electric field and an oxygen potential gradient (vacancy composition gradient) of semiconducting and initially homogeneous mixed oxides (A,B)O of the rock salt type is analysed. The usual assumption of neglecting the off-diagonal phenomenological coefficients is avoided by assuming a random distribution of cations and making use of the exact sum-rule expression that relates the phenomenological transport coefficients for this situation. The kinetic demixing analysis is then considerably simplified. We verify the analysis with Monte Carlo computer simulation. It is found that for demixing in an electric field, the vacancy composition profile is not in fact flat, but exhibits a maximum that depends on the ratio of the cation-vacancy exchange frequencies. When a vacancy composition gradient and an electric field are both applied, it is shown that unusual steady state vacancy composition profiles can result. (C) 2004 Elsevier B.V. All rights reserved.