Synthesis of Ge nanocrystals in SiO2 is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The SiO2films containing Ge nanocrystals show intense visible photoluminescence at room temperature that is very similar in spectral features to that of SiO2 containing Ge nanocrystals synthesized by other methods, such as co-sputtering and hydrothermal reduction. The dependence of the measured peak luminescence energy on the nanocrystal diameter shows a poor correlation compared to the calculated sizedependent exciton energy for Ge ‘quantum dot’ states. The measured luminescence lifetimes are much shorter than those predicted by calculated radiative decay rates for the observed size range. The photoluminescence spectra show only a weak temperature dependence. In addition, very similar photoluminescence spectra have also been observed from Xe+-implanted SiO2with damage profiles similar to Ge+-implanted SiO2. Furthermore, the luminescence has been shown to be reversibly quenched by deuterium. These results indicate that the process responsible for visible photoluminescence is not the radiative recombination of excitons in Ge ‘quantum dots’ but is instead related to luminescent radiative defect centers in the matrix or at the nanocrystal/matrix interface.