The surface glow associated with Lucalox alumina insulator under high‐voltage dc and ac stress in bridged vacuum gaps was determined to be a continuum radiation with band limits from about 220 to 500 nm with a peak around 320 nm. A second peak with very low intensity was observed at longer wavelengths. Such surface luminescence from other materials (including different types of alumina) bridging the vacuum gap was either absent or too weak to be analyzed. There is ample evidence to support the view that the observed glow with Lucalox (99.9 wt. % Al2O3) is associated with electronic transitions between quantum‐mechanical states at defect sites (either interstitial atoms or vacancies) in the crystal structure of Al2O3. These defect sites are either theF+centers (one electron trapped at an anion vacancy, i.e., Al2+ion) or theFcenters (two electrons trapped at an anion vacancy, i.e., Al+ion). The results of this investigation indicate that the observed luminous front along an insulator surface in vacuum, prior to flashover, is not necessarily indicative of a streamer‐type discharge development in the gas phase, desorbed from the insulator surface. Such a luminous front is indicative of electronic transitions at defect sites located at or near the insulator surface.