The effect of an external electric field applied, at elevated temperatures, to a thin-film diffusion couple has been studied. Thin films of epitactic Fe-oxide were deposited on {001} MgO by means of pulsed-laser deposition. The diffusion couple was then reacted at 1150 degrees C in air for 2 h in an applied field of 5 kV cm(-1). The distribution of phases after the reaction was determined using backscatter low-voltage scanning electron microscopy. In performing the analysis, images taken from the field-applied diffusion couple were compared to those of a standard diffusion couple of the same geometry with no applied field. The results showed that the application of an electric field affects both the diffusion of the cations and the interface morphology.