The formation process is described of supported microporous membranes, prepared by modification of ct-alumina supports with colloidal silica sols. The sols were prepared by acid catalyzed hydrolysis of tetraethylorthosilicate (TEOS), using different molar ratios of H2O/TEOS and of HNO3/TEOS. Membrane composites with multiple separation layers were synthesized by a conventional dip-coating method, in which process multiple layers with interconnected pores were formed by sequentially dipping a support in sol solutions in descending order of sol size, Emphasis is given to the parameters of dipping process such as dipping time and the number of dipping procedures applied, and to the preparation conditions of silica sols, in order to identify the main factor governing performance of processed membranes. It has been found that the modification procedure using several sols with different sol size is effective for preparing a multiple separation layer through which a percolation system of pores is successfully formed. The proposed process has made it possible to prepare membranes with CO2 permeation rates on the order of 10(-3) cm(3)(STP) cm(-2) s(-1) cmHg(-1), which is one to two orders of magnitude higher than those previously possible by sol-gel techniques. The ideal CO2/N-2 selectivities, determined from single-component gas permeation experiments, are 4.2 for high-quality membranes.