Well- ordered organic - inorganic mesoporous films are prepared using an original two- step synthesis and compositions up to 50 mol% methyltriethoxysilane ( MTES). These materials exhibit the three different mesostructures ( 2D- hexagonal, 3D- hexagonal and cubic) previously observed in the pure silica system with significant modifications of their domain of stability. During deposition and drying of organosilicate films, MTES monomers progressively condense at the internal surface of the silica clusters in agreement with the monomer - cluster growth model, leading to a significant reduction of the microporosity in the silica walls of the mesoporous structure.