The luminescence from a planar metal–alumina–metal structure was investigated under ac voltage application in vacuum. Two kinds of electrode contacts, i.e., with/without sputtered gold film, showed quite different optical phenomena. For the nonsputtered alumina samples, no luminescence was detected until extremely irregular light pulses resulted from partial discharges under higher voltage. While for the sputtered samples, from far lower applied voltage, there was faint but stable light emission due to electroluminescence (EL) observed. Based on the band theory of solids, we proposed a model to explain the interesting phenomena qualitatively. Taking account of the sputtering process, a marked reduction of the potential barrier between electrodes and alumina surface would result. Thus, charge carriers can be easily injected from the electrodes into the surface layer of alumina, and EL will emit due to the radiative recombination of electrons and holes. It is considered that EL has a significant influence on the developing process of surface flashover. © 2001 American Institute of Physics.