Mesostructured silicas can be obtained from silica solutions or gels containing amphiphilic surfactants as structure-directing agents. Composite materials with hexagonal (MCM-41), cubic (MCM-48), lamellar (MCM-50) and with disordered (LMU-1) topologies are formed in the system C(n)TMA(+) -SiO2-KOH-water-ethanol, where C(n)TMA(+) is an alkyltrimethylammonium cation. The first synthesis field diagram obtained at usual preparation conditions (2 d hydrothermal treatment at 110 degrees C) by systematic variation of both the surfactant (C(14)TMA(+)) and the silica concentration is presented. It reveals optimum synthesis conditions for highly ordered materials and interesting implications for the synthesis mechanism.