Positron annihilation spectroscopy (PAS) is used to study Si-rich SiO2samples prepared by implantation of Si (160 keV) ions at doses in the range 3×1016–3×1017cm−2and subsequent thermal annealing at high temperature (up to 1100 °C). Samples implanted at doses higher than 5×1016cm−2and annealed above 1000 °C showed a PAS spectrum with an annihilation peak broader than the unimplanted sample. We discuss how these results are related to the process of silicon precipitation inside SiO2.©1997 American Institute of Physics.