The cooperative self-assembly of organic–inorganic siliceous composite structures has been studied from the aspect of inorganic precursors. We reveal that the vesicular or mesostructured materials can be obtained selectively by just changing the silica sources in one templating system. For poly(ethylene oxide)-type block copolymers with either poly(propylene oxide) or poly(butylene oxide) as the hydrophobic moieties, when the other synthesis parameters are exactly the same, the use of tetramethyl orthosilicate (TMOS) as a silica source gives rise to highly ordered mesostructures, while the use of tetraethyl orthosilicate (TEOS) leads to vesicles or foams. The attenuated total reflection Fourier transform infrared (ATR-FTIR) technique is used to monitor the silicate species derived from the hydrolysis and condensation of TMOS and TEOS as a function of the reaction time. On the basis of the ATR-FTIR results, we propose a “differentiating effect” at relatively high pH (4.7) to interpret the influence of different silica sources on the self-organized composite structures. For comparison, a “leveling effect” at relatively low pH (strong acidic conditions) is revealed to explain that both TMOS and TEOS lead to the same mesostructures. Our contribution provides a feasible and designable method to synthesize from conventional ordered mesostructures to novel vesicular structures, which are significant for their future practical applications.