An analysis of thermoionic converters operating in the extinguished mode is presented. Expressions for the forward and reverse saturation output current densities, and for the open circuit voltage are derived for the first time from a single set of transport equations and boundary conditions. Agreement between theoretical and experimental results is established. It is shown that the output current density cannot exceeed a certain upper limit which depends only upon the emitter temperature and the interelectrode spacing, and is independent of the emitter work function and the cesium pressure. It is shown that, under certain operating conditions, measurements of the forward and reverse saturation output‐current densities and of the open circuit voltage can be used to infer values of the emitter temperature, emitter work function, collector work function, and electron and ion mobilities.