Silica membranes with extremely low defect concentrations have been prepared using sol-gel synthesis starting from tetraethyl orthosilicate (TEOS). An asymmetric structure is obtained by applying two silica layers on top of a gamma-Al2O3 layer, supported by an alpha-Al2O3 support. The selective silica top layers have a total thickness of 30 nm and micropores with a pore diameter similar to 5 Angstrom, determined by physical adsorption on unsupported silica membrane material. The morphology of the homogenous silica layer is analysed by FE-SEM and TEM. The transport properties of the membranes are measured in the temperature range of 50-300 degrees C and at pressure differences ranging from 0.5 to 3 bar. The membranes have reproducible high permeances (2x10(-6) mol/m(2).s.Pa) for H-2 and much lower permeances for CO2, N-2, and O-2 (similar to 10x lower), CH4 (similar to 500x lower). The silica membranes show a slight increase of permeance with increasing temperature for H-2, CH4, N-2, O-2 and a slight decrease for CO2. For the time being the pore size of the microporous supported silica membranes can be estimated best from the relation between the permeance and kinetic diameter (d(k)) of the gases used. The pore size can be derived from the value of the d(k) of the smallest gas where permeance is no more observed. The silica membranes can be applied for different types of separations : CO2/CH4 with a permselectivity (F-alpha) of 75, O-2/N-2 With F-alpha=4, H-2/CH4 with F-alpha>500 and H-2/CO2 with F-alpha=70, all at 200 degrees C. Separation factors obtained from gas separation experiments with 50/50 (vol.%) gas mixtures are very similar to permselectivities calculated from single gas permeance experiments.