The ionization time for four types of commercial Thyratron tubes has been determined for the following cases: (1) impulsive voltage applied to the anode with the grid biased positively or negatively, (2) impulsive voltage applied to the grid with the anode potential constant, and (3) impulsive voltage applied to grid and anode simultaneously. The principal factors in fixing the ionization time are (1) the tube design; (2) the mercury vapor pressure (the time lag in starting is increased and made more erratic by decreasing the vapor pressure); (3) the grid bias (increasing the bias negatively increases the time lag); (4) the steepness of the wave front applied to the system. The lags become less as the rate of building up of voltage increases. This is due partly to the higher voltages reached and partly to the greater induced grid voltages when the impulse is applied to the anode. This effect is computed for two kinds of applied voltage waves. The time between grid and anode breakdowns was measured for an FG‐41 and an FG‐17. The grid and anode currents during this interval were computed. For the particular FG‐41 used,ianode=3igrid. A table of ionization times under these various conditions is given for four Thyratron tubes (FG‐17, 33, 41, 57).