Thyratrons of FG‐41 type were tested at over‐voltage for arcback in a circuit which simulates operating conditions very closely. Memory oscillograms showed the time in the cycle at which the arcbacks occurred. At a mercury‐control temperature of 45°C, the distribution was nearly random, with a maximum at the time of highest negative anode voltage, and depending on voltage in the same manner as the average arcback rate, taken over a long period. When the mercury‐control temperature was raised to 64°C, all the arcbacks occurred at the beginning of negative voltage application. These results are interpreted on the theory recently advanced by Kingdon and Lawton, which attributes arcback to charging up of small non‐conducting particles on the surface of the anode. At low vapor pressure, where the ions remaining from the discharge have time to diffuse to the electrodes before negative voltage is applied, the charging is produced by processes such as glow discharge and field emission, which depend on voltage; hence arcback is correlated with voltage. At higher pressure, diffusion is interfered with by molecular collision, so that ions are present when negative voltage is applied. In this condition arcback occurs at the beginning of the negative cycle.