Self-assembled porous silica materials with adjustable structures and tunable pore sizes have important applications in catalysis, separation, and nanoscience. Organic cosolvents such as 1,3,5-trimethylbenzene (TMB) can be used to synthesize large pore mesoporous materials. In this study, we systematically studied the influence of the time of TMB addition on the self-assembled organic/inorganic composite structures in a nonionic block copolymer templating system. By controlling the time at which TMB is added to the system, an evolution from multilamellar vesicle to ordered hexagonal mesostructure has been observed. TMB is a swell agent in our synthesis, an increase in the delay of TMB addition can kinetically reduce the amount of TMB penetrating into the hydrophobic core of embryonic mesostructure, leading to cooperatively self-assembled vesicular and mesostructured materials with decreased packing parameters. Our results have shown that, in the simple synthesis system of traditional SBA-15 material, siliceous materials with a range of structures can be rationally designed and synthesized through the addition of TMB at different times. Such materials with tunable pore structures have potential applications as microcapsules and controlled release/delivery carriers.