In this study, phase transformation of the hexagonal mesostructure MCM-41 to the cubic mesostructure MCM-48 is examined by in situ X-ray diffraction (XRD) of the transforming mesostructure and by XRD of products from bulk transformation experiments in Parr autoclaves. Transformations were studied under conditions of high pH and temperatures between 100 and 190 °C. Heating events took place after the hexagonal mesophase had assembled, but before it had fully polymerized. On the basis of these and previous results on transformations in silica−surfactant−water and surfactant−water systems, a model is proposed to explain the expected hexagonal → cubic transformation as well as the brief existence of a lamellar phase during the transformation. Additional experiments to establish synthetic parameters for the transformation included varying the silicon alkoxide source, replacing the supernatant prior to heating, and adding fluoride or aluminum to the reaction mixture. The results, taken together, illustrate the strong cooperativity between the organic and inorganic regions in controlling the assembly of the mesostructure and provide a better understanding of the effects that control phase transformations in these systems.