Breakdown phenomena and the low‐voltage xenon arc have been studied in xenon‐filled thermionic diodes. These measurements were made at gas pressures ranging from 1 Torr to atmospheric pressure and with diodes having interelectrode spacings of 0.05, 0.1, and 0.75 cm. Most of the results were obtained on tubes with a unipotential thoriated‐tungsten cathode operating at approximately 1700°C. The results showed that breakdown in xenon leading to the low‐voltage arc can occur at voltages even lower than the first excitation potential of xenon. No minimum spacing effect was observed; the low‐voltage arc occurred for interelectrode spacings down to 0.05 cm. At 0.05‐cm spacing and a cathode temperature of 1700°C, the low‐voltage xenon arc could be induced over the pressure range 1–500 Torr. Under some conditions of pressure and spacing the low‐voltage xenon arc could be sustained at voltages as low as 1–2 V.