Pure and porous silica xerogels doped with CdS nanocrystals have been prepared by a sol-gel process. In order to determine parameters convenient for non-linear optical properties, particle size distributions were obtained by two complementary techniques : transmission electron microscopy (conventional, CTEM, and high resolution, HRTEM) and small angle X-ray scattering (SAXS). Monolithic samples having CdO concentrations varying from 5 to 20 wt % have been studied. Details are given of an image analysis technique used to study the CTEM micrographs.