The syntheses of silicas with highly ordered cubic Ia3d structure templated by Pluronic P123 (EO20PO70EO20) block copolymer surfactant and sodium dodecyl sulfate (SDS) additive in the presence of swelling agents are demonstrated. It was found that the cubic Ia3d silica forms at 25 °C when a moderate amount of a swelling agent, such as 1,3,5-triisopropylbenzene (TIPB), 1,4-diisopropylbenzene (DIPB), or 1,3,5-triethylbenzene (TEB), is added. However, 1,3,5-trimethylbenzene was not found suitable, suggesting that the success of the synthesis requires a careful selection of a swelling agent. An increase in the relative amount of the swelling agent in a limited range tends to cause an increase in the unit-cell size, while a further unit cell parameter increase can be accomplished with TIPB through a concomitant decrease in the synthesis temperature and increase in the relative amount of the swelling agent. Many of the cubic Ia3d products, including those with the largest attained unit-cell sizes, were highly ordered. When TIPB was used as a swelling agent, the products typically had unusually high mesopore volumes. The latter was largely independent of the ratio of the silica precursor to the Pluronic P123 surfactant for high quality products obtained under particular conditions, which suggests that the cubic Ia3d structure forms at a nearly constant silica-to-surfactant ratio.