Radio‐frequency oscillations observed in cesium‐filled thermionic diodes are interpreted on the basis of a model which assumes that the cesium ions oscillate in an excess negative charge potential well outside the cathode. A simplified theoretical treatment shows that the period at the onset of oscillations is linear with cathode‐anode spacing in agreement with experiment, and relates the oscillation period to the ratio of cell current to saturation emission current. It is found that the rapid decrease of oscillation amplitude as the cell current reaches a critical value can be used to measure saturation emission currents.