Mesoporous silica MCM-48 membranes were synthesized on a porous alumina support using surfactant molecules which act as template for. the polymerization of silica. Higher synthetic temperatures resulted in thicker silica wall. However, the average pore size of MCM-48 was independent of the synthetic temperature. The permeation of N-2 through the calcined MCM-48 membrane was governed by the Knudsen diffusion. There was no contribution of viscous flow, which can be observed in large pinholes. The pore size distributions of silylated mesoporous silica MCM-48 membranes were evaluated by permporometry measurements in which N-2 flux was monitored in the presence of methanol vapor. A sharp decrease in the N-2 flux at a relative vapor pressure of methanol equal to 0.5 was observed, indicating a narrow pore size distribution of the MCM-48 membrane. The corresponding Kelvin diameter of the membranes was about 2.0 nm, which is consistent with the value for the silylated MCM-48 powder calculated with Barrett-Joyner-Halenda (BJH) method using N-2 adsorption isotherm. (C) 2003 Elsevier Science B.V. All rights reserved.