Abstract In some insulating liquids containing ionizable impurities, an applied field may inhibit the convective transfer of heat between a heated wire and the fluid. This can be ascribed to the space charge which builds up in the vicinity of the wire surface under the combined action of the conductivity gradient (resulting from the temperature gradient) and the electric field. For lightly doped liquids under low stress, this space charge undergoes from the wire a resultant downward Coulomb force opposing the upward buoyancy, thus reducing the heat transfer, regardless of the sign of the applied voltage. For strongly doped liquids, a polarity is observed; this appears to be related to the electrical double layer at the interface, which depends on the ionic concentration. Conversely, the study of this effect of polarity on heat transfer might prove to be a valuable tool for understanding the interface between metals and slightly conducting fluids.