Reports results of an experimental investigation of the influence of gas desorption from electrodes on electrical breakdown in high vacuum (P=10-9 Torr). The gas composition was studied by a quadrupole mass-filter. Thermo-desorption of hydrogen from the anode was the dominant effect. A close correlation between gas evolution from the anode and changes in the slopes of Fowler-Nordheim plots for prebreakdown currents was established. Pressure bursts due to gas released from the anode at breakdown were observed.

The process of beam spreading of field-emission electrons under the action of space charges, suggested by Maitland (1961) as an explanation of vacuum breakdown initiation, is analysed. This type of space charge effect is shown to be negligible. A plausible model of breakdown initiation as a result of ions focusing within the electron beam is discussed and shown to be unreal. The effects of space charges on the initiation of vacuum breakdown are thus concluded to be very small.