Abstract Experiments have shown that the application of a DC electric field may induce both stationary and oscillatory instabilities in a plane layer of dielectric liquid heated from above. A linear instability analysis of a unipolar charge injection model in which carrier mobility varies linearly with temperature is discussed. It is shown that in general if the thermal gradient is weakly stabilising then stationary instabilities are dominant, whilst for a strongly stabilising gradient overstable modes are preferred. However, for a sufficiently small charge injection strength, electrode thermal polarity is important. Stationary (oscillatory) modes are preferred if the emitter is cooler (hotter) than the collector.