Pervaporation and gas permeation properties of microporous silica membranes made by a sol−gel methodare discussed. Defect free molecular sieving membranes are prepared by a dip coating process. The molecularsieving performance was measured and controlled based on gas permeation behavior of the membranes.The apparent activation energy for helium permeation and He/N2 perm-selectivity values were used asthe parameters for optimization of the membrane performance. The membranes with very high activationenergy for He diffusion were used for pervaporation studies with a methanol/MTBE mixture at 323 K.Separation factor values as high as 260 were measured at a total liquid flux of 0.3 kg/m2 hr. Sorptionstudies performed on corresponding silica gels revealed a separation mechanism based on diffusion ofvapor molecules. Permeation of the vapor molecules through the micropores followed an activated diffusionmechanism. The gas permeation data could provide an understanding of the pore size distribution of themembrane, and the vapor sorption and diffusion data on the size and connectivity of the membrane pores.