The role of the surfactant cetyltrimethylammonium chloride/hydroxide in the synthesis of the mesoporous molecular sieve MCM-41 was studied by powder X-ray diffraction and Si-29 magic-angle-spinning NMR. The aim was to test the hypothesis that the surfactant micelles template the formation of the solid and/or catalyze the hydrolysis of organic silicate and its subsequent polymerization. MCM-41 can be synthesized with surfactant concentrations as low as, but not below, the critical micelle concentration, which provides the first direct proof that surfactant micelles indeed template the synthesis. In the absence of the surfactant the product is invariably amorphous, and the rate of silicate polymerization increases by a factor of more than 2000 when the surfactant is added. The micelle catalysis mechanism relies on electrostatic interaction at the micelle-silicate interface and the higher silicate concentration near the interface than in the bulk.