This article reports the structural variation of SBA-type mesostructured silica formed from a mixture of tetraethyl orthosilicate (TEOS), alkyltrimethylammonium bromides, and 1,3,5-trialkylbenzenes under acidic conditions. Swollen 2D hexagonal mesophases were formed from the silica source and hexadecyltrimethylammonium bromide (C16TAB) with varying amounts of trimethylbenzene (TMB) and triethylbenzene (TEB), whereas drastic structural changes were observed with triisopropylbenzene (TIPB). Characterization by X-ray diffraction and transmission electron microscopy observation revealed that the mesophase was changed from hexagonal p6mm to cubic Pm3n to cubic Fm3m with increasing amounts of TIPB. Thus, the addition of TIPB leads to the preferential formation of spherical micelles rather than the swelling of rodlike micelles. When tetradecyltrimethylammonium bomide (C14TAB) was used, similar structural changes were triggered by smaller amounts of TIPB; however, almost no structural change was observed when octadecyltrimethylammonium bromide (C18TAB) was used. These findings provide a better understanding of the roles of 1,3,5-trialkylbenzenes in the structural control of silica-alkyltrimethylammonium mesophases.