Faradaic current measurements have been carried out on three different types of mullite: 2:1 mullite single crystals ( ⊥ ), 3:2 ceramics and 11% mullite/Mo composites. Measurements were carried out on very thin samples (60 μm) at high voltages (500 to 1000 V). Under these conditions, measurable currents were recorded even at room temperature. Results indicate notable differences between these three samples, which suggest that, although they share the same name and similar crystalline structure, binding energies and defect distributions seem to be very different. Finally, it has been seen that the excellent behaviour against dielectric breakdown of ceramic mullite does not hold for single crystals or mullite based cermets.