A description is given of a method of measuring the effective deionization time in thyratrons, i.e the time that must elapse, after arc current has ceased, before the grid regains control. In this method, voltage is applied to the anode at a known rate and at any desired interval after the passage of an arc-current pulse. The duration of the pulse, a few microseconds, is negligible compared with the deionization times being measured. The interval before the anode voltage is reapplied is reduced until the loss of grid control indicates that insufficient time has been allowed for deionization. A study of the decay of ionization, by observations of the ion current to the negatively charged grid, shows that grid control may be regained before deionization is quite complete.The method is illustrated by the results of comparative tests showing the variation of deionization time with changes in grid design, and in the nature and the pressure of the gas or vapour filling.