Formation of nanocrystalline rare earth (RE) silicates (RE = Y, Nd) inside or at the surface of amorphous SiO2 matrix upon heat treatment in air was studied by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). RE-doped SiO2 samples were prepared by the sol-gel or impregnation method. The structure evolution of the silicate phase depends on the preparation method and RE2O3:SiO2 molar ratio. For a Nd2O3:SiO2 molar ratio below 1:2, the formation of a superficial silicate, similar to Ce-6[Si4O13][SiO4](2) reported recently, was observed in the temperature range 900-1100degrees C. (C) 2003 Elsevier Science B.V. All rights reserved.