The microstructure of a heterogeneous SiO2 submitted to a depolymerisation process is studied using Transmission Electron Microscope (TEM), Environmental SEM (ESEM), and X-ray diffraction (XRD). With ESEM the formation of micro domain induced by the dissolution phenomena is shown. XRD shows the formation of a halo that is associated with the formation of amorphous phase. The parameters "position and FWHM" of the halo, enabled us to show the evolution of the disorderly phase when the reaction progresses. The hypothesis of formation of nanoparticles with different structural states was confirmed by the TEM. (C) 2010 Published by Elsevier B.V.