The formation mechanism of the hexagonal, MCM-41, and the lamellar, MCM-50, mesoporous materials, prepared at room temperature with the surfactant cetyltrimethylammonium chloride (CTAC) and tetraethylorthosilicon (TEOS), was studied by in situ EPR spectroscopy using the spin probe 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethylpiperidinyloxy iodide (CAT16). This probe has a structure similar to that of the surfactant molecules with the nitroxyl radical situated at the head group. Accordingly, it probes the interface between the organic and inorganic phases during the formation of M41S materials. The EPR spectrum of CAT16 in the reaction gel, prior to the addition of TEOS, consists of a superposition of two subspectra due to spin probe molecules in micelles and in the aqueous phase, respectively. For a gel composition which forms MCM-41, the addition of TEOS leads to a gradual transformation of the micellar subspectrum into a characteristic rigid limit spectrum. This observation provides direct evidence that micellar structures present in the initial reaction mixture serve as precursors for the final mesoporous product. The temporal evolution of the spectrum is characteristic of an isotropic system undergoing a gradual increase in the microviscosity. The isotropic nature of the spectrum is a consequence of the specific geometry of the CAT16 head group and its motion in the interface region. Comparison of the temporal evolution of the EPR spectrum with that of the X-ray diffraction pattern indicates that the hexagonal long-range order is formed already 5-8 min after mixing the reagents, whereas the formation of the inorganic phase, which is apparently responsible for the slowdown of the spin probe motion, is considerably slower (> 1.5 h). The latter process begins only after a critical amount of TEOS is added to the mixture. These results are consistent with a mechanism whereby the addition of TEOS initially forms clusters of rodlike micelles coated with silicate anions, followed by the condensation of the silicate anions at the interface to yield the final product. By monitoring the peak height of the central EPR line, phenomenological kinetic profiles of the reaction were obtained. These curves were quite different for MCM-41 and MCM-50 and they provide qualitative information regarding the sequence of transformations which occur during the reaction. Specifically, these curves show that while no intermediate phases occur during the formation of MCM-41, several phase transformations take place when MCM-50 is formed and the reaction is significantly slower.