Tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) have been used to prepare hybrid SiO ₂ -based membranes. These self-supported materials were obtained from controlled polymerization reactions for various TEOS/MTES molar ratios ensuring the achievement of crack-free disks 8 cm in diameter and 10–40 µm in thickness. The rheological behaviour of precursor solutions was studied and gelling times were determined. The whole process, from starting solution to xerogel, was followed by FTIR spectroscopy, viscosity measurements and multinuclear solid-state NMR, and is discussed in terms of the hydrolysis–condensation kinetics of tetrafunctional and trifunctional silicon alkoxides. Density, shrinkage, elastic modulus (E), modulus of rupture (MOR) and elongation at break were all determined and related to preferential structural arrangements of networks according to the TEOS/MTES ratio.