This work reports a systematic study of ordered mesoporous silicas (OMSs) synthesized with and without hydrothermal treatment at 373 K for a series of surfactants of different alkyl chain length (from C10 to C18). For these samples nitrogen adsorption and small angle X-ray scattering (SAXS) data were measured to characterize their adsorption and surface properties. Namely, nitrogen adsorption isotherms were used to evaluate their specific surface area, pore volume and pore size distribution, whereas SAXS data provided information about their structural ordering. It is shown that while the room temperature synthesis afforded OMS samples with cubic MCM-48 structure, an additional 5-day hydrothermal treatment of these samples at 373 K caused their transformation to MCM-41 (two-dimensional hexagonal structure) and improved their pore uniformity, which was manifested by reducing the width of pore size distribution. (c) 2005 Elsevier B.V. All rights reserved.