The hexagonal, silica-based, mesoporous material SBA-15 is prepared using poly(ethylene oxide)-poly(propylene oxide) -poly(ethylene oxide) block co-polymer (Pluronic P123, PEO20-PPO70-PEO20) as a template and tetramethyl orthosilicate (TMOS) as a silica source. This work focuses on the investigation of its formation on the molecular level, with emphasis on the early stages of the reaction, when the interaction between silica precursors and the Pluronic micelles occurs. This was achieved using in situ X-band electron paramagnetic resonance (EPR) spectroscopy, in combination with electron spin-echo envelope modulation (ESEEM) experiments of Pluronic spin probes with different PEO and PPO chain lengths. In these Pluronic spin probes, the nitroxide spin label is located at the end of the PEO chain, which places them at different regions of the micelles. In the Pluronic micelles, the PPO chains define a hydrophobic region that is referenced as the core, whereas the more-hydrophilic PEO chains form the corona region. In the ESEEM experiments, the reaction was conducted in D2O and it was quenched at different times by rapid freezing to 77 K. The 2 H modulation depth (k(H-2)) was followed, as a function of the reaction time. Four different systems, which were designed to probe the evolution of the reaction at three different regions of the Pluronic micelles, were examined. By comparing the ESEEM results with the in situ continuous wave (CW) EPR measurements of the different spin probes, four stages were detected. The first occurs within the first 5 min and is characterized by a large increase in the D2O/OD density in the vicinity of all spin probes used, whether located in the core, the core/corona interface, or the corona/water interface. This was attributed to fast hydrolysis of the TMOS where hydrolyzed TMOS and water penetrate into the corona from the original location of the hydrophobic TMOS within the core. The second stage, which lasts similar to1 h, is characterized by a moderate reduction in the D2O/OD density at the core/corona interface, attributed to silica polymerization. During the third stage (similar to1 h), the D2O/OD density decrease continues but is felt mainly in the corona region. The results show that the silica polymerization propagates outward from the core/corona interface.