[en] Either of two distinctively different surface flashover mechanisms may lead to the electrical breakdown of an insulator under high voltage stress in vacuum. While the more familiar cathode-initiated mechanism propagates toward the anode and depends on secondary electron multiplication on the insulator surface, anode-initiated flashover propagates in the opposite direction and appears to involve processes related to bulk breakdown. The study of anode-initiated flashover may therefore help to elucidate the physics of the treeing mode of insulation failure. In addition, anode-initiated flashover very likely limits the electric field that can be withstood by conventionally designed insulators having surfaces inclined to avoid electron multiplication. In the work reported here, a variety of dielectric materials were subjected in vacuum (10^-3 to 10^-4 Pa pressure) to high voltage steps having 3 ns risetime. Damage patterns on the insulator surface characteristic of anode-initiated flashover occurred with various experimental arrangements having in common high electric fields directed into the surface of the insulator. The flashover mechanism was studied by detecting emission current from the insulator and by examining the surface damage with the aid of scanning electron microscopy. A model of the flashover mechanism based on these observations is proposed