A plasma model and a close‐spaced model are used to analyze the detailed variation of the open circuit voltage of the cesium diode. The plasma model open circuit voltage expression is compared to one derived by Lewis and Reitz in which the ion contribution to the net electron current is neglected. For either model the main features of the open circuit voltage at emitter temperatures less than 1800°K arise from the complex ion and electron emission properties as a function of emitter temperature and cesium pressure. Although at higher emitter temperatures the plasma model open circuit voltage becomes independent of the detailed surface characteristics of the emitter, the two models continue to give similar numerical results. Calculations based on these models and incorporating the data of Langmuir agree qualitatively with the sinusoidal and discontinuous open circuit voltage variations observed by Piddet al., as well as the anomalous load characteristic observed by Wilson.