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Early breakdown effects have been measured in relatively high‐pressure thermionic diodes under given conditions of pressure and anode spacing. Apparent breakdown conditions are obtained at anode voltages considerably lower than the ionization potential of the gas. Measurements on xenon‐ and mercury‐filled thermionic diodes, under space‐charge conditions of operation, are presented which show that this breakdown voltage seems to be associated with the first excited state of the gas. Experimentally, early ignition occurs in diodes with a fixed cathode‐anode spacing in a limited pressure range only. A theoretical explanation for this behavior is based on the model that, in the limited pressure range, electrons traversing the cathode‐anode space can pick up almost all the energy of the applied voltage. In addition, data are presented showing that early breakdown in an inert gas‐filled diode can occur at voltages much lower than the first excited state. This has been accomplished in thermionic diodes exposed to the radiation of a nuclear reactor. A qualitative explanation for this behavior is proposed.