Three different hybrid organic/inorganic solid acid catalysts were prepared by grafting perfluorosulfonic acid onto amorphous silica gel by covalently anchoring beta-sultone onto the hydroxyl groups on the silica surface. The anchored sulfonic groups have been characterized by thermal analysis, infrared spectroscopy, and photoelectron spectroscopy (XPS). Particularly, the XPS technique has proven to be an extremely powerful tool to determine the nature and chemical state Of Sulfur and carbon, as well as the surface density of the acid groups. The hybrid organic/inorganic systems functionalized with sulfonic acid groups were tested in the esterification reaction of acetic acid with methanol in liquid phase. Activity results obtained at 333 K and using an initial molar ratio AcOOH:MeOH = 1: 1 revealed that the acid-functionalized, nonsilylated catalyst reaches acetic acid conversion of similar to 50%, which contrasts with the much lower level (similar to 30%) reached by a commercial Nafion silica composite sample. Treatment of the catalysts in methanol at 333 K under stirring for 48 h emphasized that only the acid-functionalized and silylated sample retains some of the sulfonic groups.