The decomposition mechanism of block copolymer templates inside as-synthesized mesostructured solids has been systematically studied using solid-state 1H magic angle spinning nuclear magnetic resonance spectroscopy, thermogravimetric analysis, and high-vacuum Fourier transform infrared spectrometry. It is shown that there exists hydrogen-bonding interaction between silanols and block copolymers at the inorganic–organic interface in the self-assembled as-synthesized mesostructured solids, which plays an important role in protecting the surfactants against decomposition during the high-temperature hydrothermal treatment process. Increasing silanol concentration can enhance the hydrogen-bonding interaction and thus shows better “protection” effect. Moreover, the thermal decomposition of the block copolymer in as-synthesized mesostructured solids in air commences at higher temperatures compared with that in acidic solution or in air, providing further evidence in support of the silanol protection mechanism.