The current‐voltage characteristics of high‐pressure (0.25–300 mm) rare‐gas diodes have been measured. Data have been taken on diodes containing xenon, argon, and helium as the ambient gas, and tungsten, tantalum, rhenium, thoriated tungsten, and an oxide cathode as filamentary cathodes. Some unexpected phenomena have been observed and some of these general results can be summarized as: (1) The current vs voltage characteristics of argon‐ and xenon‐filled diodes, at gas pressure above 1 mm, do not obey any space‐charge law if the cathode temperature is above 2400°K. (2) The above violation of space‐charge relations in argon‐ and xenon‐filled diodes is found with tungsten, tantalum, or rhenium filaments, which can be operated above 2400°K. When thoriated tungsten or the oxide cathode, which operate below 2400°K, are used as filaments, the current‐voltage characteristics of the diode follow a space‐charge relation (current‐voltage characteristics independent of temperature). (3) The current vs voltage characteristics of helium‐filled diodes, however, do obey the space‐charge relations at all filament temperatures available with the present cathode materials. (4) Some very unusual early breakdown phenomena (breakdown at potentials below ionization potential) were observed in these high‐pressure rare‐gas diodes employing a hot cathode. These data can be explained qualitatively by postulating the existence of thermally generated xenon or argon gas ions at temperatures in the range of 2400°K. An attempt will be made to justify this assumption by a semiquantitative theoretical treatment based on Saha's thermal ionization theory rather than the surface ionization theory of Langmuir and Kingdon.