A cesium vapor diode was operated both before and after cesium fluoride was introduced into the interelectrode space. The electrodes were plane parallel, had a cross‐sectional area of 2 cm2, and were made of molybdenum having a polycrystalline surface. The interelectrode spacing was varied from 0.01 to 10 mm. The cesium fluoride vapor additive caused an increase in the optimized output power density. For a given emitter temperature this enhancement increased with spacing until the spacing reached about 0.89 mm, after which it became uniform. The enhancement decreased with an increase in emitter temperature. The increase averaged about 20% for a combination of these cases. When all parameters were held constant except the cesium fluoride reservoir temperature, an increase in the additive vapor pressure caused the enhancement to decrease. The optimum collector temperatures decreased up to 2% after the cesium fluoride addition. The optimum cesium vapor pressures were decreased by about 15°K after the CsF addition.