Electroluminescence (EL) phenomena reveal some important information about the electrical ageing of solid insulating materials in discharge-free condition. The EL emission from the surface of several insulating polymers with a planar electrode system was investigated under ac electric field in vacuum. It was observed at a voltage as low as /spl sim/1 kV. Its time-resolved distribution under stepped voltages, the intensity as a function of ac, positive and negative half-rectified ac voltages, the phase-resolved and spectral distributions are studied in detail. Based on the band theory of solid, a new model has been proposed to describe the EL phenomena observed, resulting from the radiative recombination of electrons and holes injected in the surface states. The experimental results and theoretical analysis also support a new approach to understanding the surface and interfacial phenomena in the field of electrical insulation.