A strong heterogeneity of composition was found in all the silica-aluminas prepared by sol-gel methods that we have examined. The Si/Al ratio at 50 to 75 points in each sample was determined by EDS X-ray spectroscopy using an analytical electron microscope. The area measured at each point was smaller than 10(3) nm(2). Relative fluctuations of the Si/Al ratio ranged from 10% to 20% for the most homogeneous samples, up to a factor of 100 in other cases. Ln some samples particles were found which contained solely silica. We have examined on the one hand a series of laboratory samples prepared by the co-hydrolysis of alkoxides over a large range of composition and, on the other hand, industrial catalysts of very similar compositions. The latter ones were prepared by various sol-gel methods probably based on using precursors other than alkoxides. impurities were also analysed. On the whole, the industrial samples are often more heterogeneous than the laboratory ones, although one of them was almost as homogeneous as the laboratory samples. Most of the industrial catalysts contained less impurities than those prepared in the laboratory.