Positron annihilation spectroscopy was conducted to study defects and Si nanoprecipitation in sputter-deposited silicon oxide films. For as-deposited SiO0.9 and SiO1.9 films, Doppler broadening spectra are strongly influenced by the type of paramagnetic defects (P-b or E'). However, the disappearance of these defects after annealing at 1050 degreesC in a vacuum cannot account for the corresponding change of the Doppler broadening. In annealed film of SiO1.9, positronium formation is reduced from defect-free SiO2 because of the presence of Si precipitates; meanwhile, in annealed film of SiO0.9 with nanocrystalline Si (nc-Si), positron annihilation with high momentum electrons is enhanced as a result of efficient positron trapping at the nc-Si surface with oxygen. (C) 2003 Elsevier B.V. All rights reserved.