Pure silica MCM-48 and MCM-41 materials were prepared using GEMINI surfactants, with the general formula [CnH2n+1N+(CH3)(2)-(CH2)(s)-N+(CH3)(2)CmH2m+1].2Br(-), abbreviated as GEM n-s-m. The alkyl chain length (n,m) determines the pore size, whereas the length of the spacer (s) influences the crystallographic phase formed. A spacer length of 10-12 C yields the cubic MCM-48; smaller spacers favor the hexagonal MCM-41 phase. A hydrothermal treatment is introduced as an intermediate synthesis step, which strongly improves the quality and stability of the calcined materials. The hydrothermal treatment reduces the synthesis time of high-quality MCM-48 to only 2 days. Both GEM 18-12-18 and GEM 16-12-16 yield excellent MCM-48, with a surface area in the range 1200-1600 m(2)/g and pore volumes exceeding 1.2 mL/g. The maxima of the very narrow pore size distributions are found at r(p) = 13.1 and 12.2 Angstrom, respectively. The GEM 16-10-16 and GEM 18-10-18 surfactants also yield MCM-48, but the quality of these materials is Fewer. The GEM 16-8-16 finally yields MCM-41.