The effects of a treatment based on high-pressure water vapor annealing (HWA) on nanocrystalline porous silicon (PS) and electrochemically oxidized PS have been investigated in terms of the photoluminescence (PL) efficiency and stability, electron-spin-resonance and infrared spectroscopy. The treatment produces highly efficient and stable luminescent nanocrystalline Si layers emitting red-orange light typical of PS. The PL external quantum efficiency reaches 23% at room temperature. Electron-spin-resonance and infrared absorption analyses show that the HWA treatment provides Si nanocrystals surrounded by a thin layer of oxide with an extremely low non-radiative defect density at the Si/SiO2 interface. This causes a drastic enhancement in the PL efficiency associated with a high reduction of non-radiative recombinations and a strong localization of excitons in Si nanocrystals. As a practical approach, the HWA technique is very useful for fabrication of efficient and stable optoelectronic nc-Si devices. (c) 2005 Elsevier B.V. All rights reserved.