Ge nanocrystals embedded in bulk SiO2 glasses were prepared by combining the sol-gel process and the heat treatment in H-2 gas. The size of Ge nanoparticles decreases with a reduction in content of Ge in the starting materials. Silica gel glasses doped with Ge nanoparticles showed a strong room-temperature photoluminescence (PL) with peaks at 568, 607, 672, 722 and 775 nm. The peak position of PL spectrum scarcely depends on the Ge/Si ratio. However, the PL intensity increases sharply as the Ge/Si ratio is reduced. The PL arises from the Ge clusters of diameter of < 1-2 nm with a molecular character instead of nanocrystalline Ge with the diamond structure. (C) 2003 Elsevier B.V. All rights reserved.