The formation of mesostructured silica was investigated using acidic conditions with tetraethoxysilane (TEOS) as the silica precursor and cetyltrimethylammonium bromide (CTAB) as the structure directing agent. A synthesis mixture of composition 2.2TEOS ∶ 1.1CTAB ∶ 75HCl ∶ 1000H2O gave a well ordered material exhibiting a range of particle shapes from extended rope-like structures of hexagonal cross-section to gyroid structures. Addition of monomethyltriethoxysilane (MTES) or tetrahydrofuran (THF) to the synthesis mixture gave rise to spherical particles. Studies of particle development showed initial growth of gel-like particles within which the mesostructure developed. A new model is proposed for the formation of mesostructured precipitates in which (1) growth of inorganic oligomers gives rise to a thermodynamically unstable mixture, (2) phase separation occurs giving droplets of a second liquid (or liquid crystal) phase rich in oligomer and surfactant, (3) the droplets are colloidally stabilized, (4) microphase separation occurs within the droplets, giving rise to a mesostructure and (5) further polymerization fixes the mesostructure.