Organic−inorganic hybrid mesostructured silicas are prepared, without stirring (at pH = 0.0) and with stirring (at pH = 0.6) to obtain isolated and gyroidal-like attached silica grains with incorporated triblock P123 poly (ethylene oxide)-poly (propylene oxide)–poly (ethylene oxide) copolymers. Selected hybrids are submitted to various thermal and aging treatments for studying hydrolysis and oxidation of PEO and PPO chains. Two complementary spectroscopic techniques are used: near-infrared (NIR) with measurements performed in diffuse reflectance, as well as 1H and 13C liquid- and solid-state nuclear magnetic resonance (NMR) to study the mobility and stability of PEO and PPO chains. With precipitated fresh hybrids, the ratio of PEO over (PEO + PPO) units is lower (30% ± 5%) than in theory. This ratio also decreases depending on temperature when a hydrothermal treatment is applied. An acidic hydrolysis involving both PEO and PPO chains, with generated fragments eliminated by washing with water and/or synthesis liquor, is then demonstrated. For comparison, another hybrid obtained by spray drying via evaporation-induced self assembly mechanism is also studied. Its NMR signals are much more stable than those of the samples prepared by precipitation. Depending on aging time, thermal history, and materials processing, the textural properties and template decomposition inside mesostructured silica are found significantly different. The influence of the hydrolysis and oxidation of the template onto final materials texture is likely to be as important as the processing conditions.