It has been found in rectifier and ignitron practice that arcback can occur after deionization of the discharge path at such vapor pressures where no self‐sustaining glow can develop. Experiments to study this kind of an arc breakdown were made in Hg vapor vessels between cold electrodes at voltages between 10 and 60 kv. It appeared that the start of arc breakdown was not dependent on the Paschen law. At the voltages used and distances between the electrodes of less than 50 cm arc breakdown always started at PD values which were much lower than those required to sustain a glow discharge. This is contrary to earlier experience at lower voltages. With rising voltage arc breakdown frequency increased in an exponential manner. Changes in vapor pressure and electrode spacing had no definite effect on arc breakdown frequency as long as glow discharge conditions were not approached. Changes in partial air pressure did not affect arc breakdown frequency if air pressure was below 10 percent of mercury vapor pressure. Heat treatment of the cathode (graphite ``anode'') has a large effect in reducing arc breakdown. Even well heat‐treated ``anodes'' show seasoning effect, i.e. steady decrease of arc breakdown frequency after initial maximum. Phenomena on cathode surface are believed to be responsible for formation of arc breakdown.