The structures and relative Lewis acidities of the catalytic sites of four aluminum-modified silicas have been studied by means of theoretical calculations on model compounds, as well as by spectroscopic and catalytic experiments. Theoretical calculations indicate that the silica treated with Et2AlCl has aluminum sites that are more acidic than those of AlCl3. The thermic treatment of the aluminum-modified silicas leads to the appearance of new, very strong acidic sites, as shown by infrared and reactivity experiments. A possible structure for these sites is proposed on the basis of the theoretical calculations.