An experimental and theoretical study is made of current nonsaturation phenomena in cesium vapor, thermionic diodes. It is shown that ion flow from the plasma to the emitter electrode produces a significant nonsaturation effect. It is also shown that this ion current produces a high‐sheath field at the emitter which accentuates the nonsaturation through the Schottky and anomalous Schottky effects. These effects are incorporated into an analysis of diode behavior. It is shown that the expected diode behavior due to ion‐current and Schottky effects is consistent with available experimental data.