Electroluminescence (EL) from p-type Si during anodic oxidation under galvanostatic conditions in ethylene glycol solutions of KNO3 with and without different concentrations of chloride ions was measured to investigate the relation between the EL behavior and the defective structure of the silica film formed on Si by anodic oxidation. The measured broad EL spectra had two peaks at 370 nm and 670 nm. The EL peak intensity of the 670 nm peak, rather than that of the 370 nm peak, was enhanced significantly by the addition of chloride ions to the solution, from which it is deduced that the EL source of 670 nm is associated with point defects which are increased by the incorporation of chloride ions in the film. Comparison between the film thickness measured with ellipsometry and the total electric charge required for anodic oxidation revealed that the current efficiency for film growth is only 1% and the majority of the anodic current is electronic, due to electrons injected into the conduction band of the silica film from solution. The relation between EL peak intensities and electrode potential suggested that the EL is operative in an impact ionization-electron avalanche mechanism, where electrons accelerated by a high electric field (2 x 10(7) V cm-1) in the film come into collision with the EL source. The origin of the EL peaks of 670 nm and 370 nm is discussed on the basis of the EL mechanism using an electronic band model of the Si/SiO2/solution system. The breakdown of the silica film due to chloride ions was accompanied by abrupt decreases in the EL peak intensities as well as a rapid potential drop, indicating that EL can be used to monitor breakdown of the film.