Measurements of the prebreakdown currents have been made for highly polished stainless‐steel electrodes in an ultrahigh vacuum (UHV) at 10−9Torr as a function of electrode separation in the range 0.5–3.81 mm. Three sets of electrodes are employed to investigate the dependence of the prebreakdown current on the radius of curvature of the electrodes and the electrodes‐insulator distance. The breakdown potential is measured in UHV gaps over the gap length range 0.25–4.57 mm and in hydrogen, in the pressure range 3 × 10−9−1.5 × 10−2Torr, over the gap length range 0.38–0.76 mm. It has been found that the breakdown voltage, the electric field enhancement factor, and the prebreakdown current are independent of electrode‐glass‐insulator distance in the range 1–12.65 mm for a fixed gap length and a fixed radius of curvature of the electrodes. On the other hand, increasing the radius of curvature of the edge of the flat electrodes, at a fixed gap length, results in (i) large increases in the prebreakdown currents, (ii) increase in the electric field enhancement factor at the cathode, and (iii) a decrease in the breakdown voltage. The addition of hydrogen causes an increase in both the prebreakdown currents and the field enhancement factor, at a fixed gap distance. The field enhancement factor at the cathode is determined from the Fowler‐Nordheim field emission theory and was found, in all cases, to increase initially with increasing gap length until saturation is reached, and therein it remains constant. The breakdown potential of vacuum gaps is found to follow a linear field‐emission dependence on gap separation up to 1 mm. At higher gaps a transition to the Cranberg clump mechanism is observed with a dependence law on distance to the power of 0.45. The critical electrical breakdown field at the cathode in high vaccum is determined to be (7.8 ± 1.3) × 109V m−1.