Small-angle neutron scattering (SANS) studies indicate that oil-in-water microemulsions, consisting of aqueous HCl, the nonionic block copolymer surfactant Pluronic P123 (poly(ethylene oxide)-block-poly-(propylene oxide)-block-poly(ethylene oxide), EO20-PO70-EO20, M-av = 5800), 1,3,5-trimethylbenzene (TMB, oil), and ethanol (cosurfactant), are novel colloidal templates that direct the synthesis of mesoporous silica with well-defined ultralarge pores. The sizes of the microemulsion droplets can be controlled by the TMB concentration and by temperature. The microemulsion droplet sizes and the cell sizes of the mesostructured cellular foam (MCF) materials increase linearly with the cube root of the TMB concentration. Increasing the temperature from 40 to 80 degrees C expands the droplet sizes, which is similar to micellar solutions of Pluronic surfactants in the absence of oil. Ethanol acts as a cosurfactant, increases the TMB solubility of the P123 micelles, and enables swelling of the P123 micelles. Low concentrations of NH4F (8 x 10(-3) mol/L) show no significant effect upon the nature of the microemulsions. The polydispersities of the droplet sizes range from 11% to 21%. The microemulsion templates reported in this paper are considered as a valuable addition to existing colloidal templates that direct the synthesis of porous materials. The benefits of the microemulsion templates are (i) their easy preparation by simply mixing water, surfactant, oil, and a cosurfactant, and (ii) the synthesis of ultralarge-pore mesoporous materials with narrow pore size distributions without the need for further processing.