Organized mesoporous silica (OMS) constitutes new materials with many potential applications. These materials with very well organized pores can be obtained by lowering the pH of precursor systems, i.e., highly alkaline silicate solutions containing cationic surfactants such as cetyltrimethylammonium bromide (CTAB) or chloride (CTAC). The precursor systems of OMS have been investigated prior to precipitation by in situ techniques. 29Si liquid-state NMR spectroscopy has been used to determine the type of silicate species in the investigated systems and how it is affected by the presence of the surfactant. Fluorescence probing techniques with the fluorescent probes pyrene and dipyrenylpropane have been used to study the variation of properties of the CTAB and CTAC micelles upon successive additions of a large excess of base (sodium hydroxide or tetramethylammonium hydroxide) and silicate (water glass or octameric species Si8O208-) as to generate the precursor systems that have the compositions that are used...