The addition of a second component to the cesium gas phase of a thermoionic generator could increase its efficiency by reducing the work functions of the electrodes.The different possible additives were screened and hydrogen, since it appeared to be the most promising, was chosen for further study.The work function reduction of cesium‐covered nickel, molybdenum, and tungsten due to the presence of H2in the gas phase ranges from 0 to 0.5 eV. An equilibrium condition can be established between the surface and the gas phase to make the low work function surface stable at elevated temperatures.