- A novel family of mesoporous molecular sieves (M41-S) has recently been reported by scientists of Mobil Oil Research and Development. During its synthesis a base- or acid-catalyzed polymerization of inorganic compounds (silica in most cases) takes place around surfactant micelles that serve as a templating agent. At the end of the synthesis, a silica matrix imbedded by micelles, namely, a mesoporous material, is obtained. Its mechanism of formation is not yet completely understood. We have studied the mechanism of formation and the phase transition sequence occurring during the synthesis of bicontinuous cubic mesoporous phase. It was found that the changes in the surfactant packing parameter, dictated by the time- and temperature-dependent reaction parameters, result in a hexagonal → lamellar → hexagonal → cubic phase transition sequence. In this article, we present a small angle X-ray scattering (SAXS) in situ study of a new preparation procedure of the bicontinuous cubic (Ia3d) phase in which an organic base is used as a reaction catalyst. The coexisting hexagonal and lamellar phases detected at an early reaction stage could indicate their importance in the formation of a cubic (Ia3d) mesophase. We found that the final cubic phase is formed only when heat is applied. The cubic phase is formed by the collapse of a preceding hexagonal phase, where a given hexagonal plane (1 0 0) evolves to a cubic plane (2 1 1) having the same interplanar distance.