A bakeable, ultrahigh vacuum, time-of-flight mass spectrometer has been constructed in order to investigate, for the first time, the nature of the ion species formed prior to the flashover of a parallel plate diode. 1 During operation, the field in the gap of the diode is maintained approximately 5% below that required for visible flashover. The anode is biased at some positive potential, V B . When a high voltage pulse, −V p , is applied to the cathode, the ions formed at the anode surface or in the gap of the diode are accelerated and pass through apertures in the cathode, striking an electron multiplier detector one meter away. The time interval between the resulting signal amplified and displayed on an oscilloscope, and the beginning of its sweep triggered by the pulse applied to the cathode is a direct measure of the ion's travel time, T. If the duration of the cathode pulse is long compared to the travel time of the slowest species in the gap of the diode, and if an ion's initial kinetic energy is zero, its identity can be determined from the expression: m/n (amu) = 0.196/L 2 V B [1 − d/D(1 + V P /V B )]T 2 where L is the cathode to detector distance, d is the distance from the species formation position in the gap to the anode, and D is the gap spacing. Here all distances are expressed in meters, all voltages in kilovolts, and the travel time in microseconds. Since ions will only be detected for O ≤ d/D (1 + V P /V B ) ≤ 1 the extent of ion production in the gap can be probed by varying the ratio of the pulse-to-bias voltage.