We have studied the structural, electrical and optical properties of MOS devices, where the dielectric layer consists of a substoichiometric SiOx (x 1000 °C) to induce the separation of the Si and the SiO2 phases with the formation of Si nanocrystals embedded in the insulating matrix. We observed at room temperature a quite intense electroluminescence (EL) signal with a peak at about 850 nm. The EL peak position is very similar to that observed in photoluminescence in the very same device, demonstrating that the observed EL is due to electron-hole recombination in the Si nanocrystals and not to defects. The effects of the Si concentration in the SiOx layer and of the annealing temperature on the electrical and optical properties of these devices are also reported and discussed. In particular, it is shown that by increasing the Si content in the SiOx layer the operating voltage of the device decreases and the total efficiency of emission increases. These data are reported and their implications discussed.