Single-phasic aluminosilicate sols were synthesized by mixing aluminum nitrate solutions with nanoparticyulate SiO2 sols. Stable aluminosilicate sols were obtained only when the pH values of SiO2 sols used initially and Al contents were below 2.0 and 20 mol% respectively. From the isoelectric point measurements. the surface of the aluminosilicate particle was found to be more negatively charged than that of SiO2. A Si-O-Al{IV} bonding model in which the corrdinatino number of all Al ions bonded to Si ions was 4 hrs been developed. The specific surface area of aluminosilicate membranes was weakly dependent on the heat-treatment temperature while was sensitiviely changed from 500 to 300 m2/g as the content of Al increased from 5 to 20 mol% A defect-free aluminosilicate composite membrane could be synthesized by the pressurized sol-gel coating techni-que using the aluminosilicate sol (0.3mol/ι) containing 10 mol% of Al made from the SiO2 sol of pH 1.8 The gas transport through the coated layer was Knudsen flow and the N2 permeability was 8.2$ imes$10-7 mol/m2.s .Pa The thermal stability of the composite membrane was excellent up to 100$0^{circ}C$