Hybrid organic-inorganic silica-based materials containing a labile bridging organic unit have been prepared. The elimination of the organic fragment is possible under very mild conditions at room temperature in the presence of a small amount (2%) of fluoride ion as catalyst. The silicas obtained after chemical treatment have been compared with those obtained by a sacrificial route (oxidation at 600°C in air). The mild cleavage of Si–C bonds eliminates the organic units and generates mesoporous silicas with a very narrow pore size distribution (45 Å). In contrast, the high temperature air oxidation gave a microporous silica with a wide dispersion of pore diameters. There is a big difference between the sizes of the pores and the size of the organic molecule eliminated. A discrepancy between the sizes of the pores in silicas resulting from the chemical elimination and the sacrificial oxidation route is also observed. A rearrangement of the hybrid network to form a pure silica network takes place during the elimination of the organic moities.