The applicability of magnetic tunnel junctions strongly depends on the electrical properties of the oxide barrier. The height and thickness of the energy barrier determines the resistance-area product of the junction, which is an important factor in the application of microstructured devices, such as MRAM. Previously the barrier height has been determined in an indirect way, namely by fitting the Simmons (1963) or Brinkman (1970) equation to the current-voltage characteristics of the junction. We have used a photoconductance set-up with which the transport properties across the insulating layer can be investigated. For the first time this technique has been applied to magnetic tunnel junctions, so that the potential step at the barrier/electrode interfaces can be determined in a direct way.