Electro-optical measurements of the electric field along solid insulator/vacuum interfaces have been made to determine the mechanisms associated with fast (ns) insulator flashover. Data showing the temporal and spatial performance of the insulator surface fields prior to and at flashover are presented. The results show that the intergap electric field is modified in a manner consistent with positive surface charging by field emitted electrons. The data also show that the enhanced intergap fields are reduced prior to flashover. Mechanisms consistent with this behavior are postulated.