gamma-Al2O3 composite membranes have been modified with microporous silica layers to improve the separation factor of CO2 to N-2 From the analysis of micropore volume fraction and CO2 adsorption behaviour of SiO2 unsupported membranes, it was found that the SiO2 membrane layer feasible to separate CO2 could be obtained from a sol prepared by hydrolysis of tetraethyl orthosilicate in aqueous nitric acid solution (acid concentration of 0.001 M and sol pH of 2.0). The unsupported membrane prepared from this optimum sol had a micropore volume fraction of 0.85 and CO2 adsorption amount of 27 cm(3)(STP)g(-1) at 0.1 MPa and 25 degrees C. Defect-free silica modified gamma-Al2O3 membranes could be synthesized by dipcoating or pressurized coating from outside the support. The CO2/N-2 Separation factor of these membranes varied severely with the separation process parameters, such as transmembrane pressure, stage cut and CO2 concentration in feed gas. gamma-Al2O3 membranes modified by dipcoating and pressurized coating had a CO2/N-2 separation factor of 2.4 and 1.45, respectively, at Delta P = 0.3 MPa, stage cut = 0.1, and 25 degrees C for a CO2 feed gas mole fraction of 0.5. The CO2/N-2 separation factor at 25 degrees C decreased with increasing heat-treatment temperature. The main mechanisms of CO2 permeation through silica modified membranes were surface diffusion and Knudsen diffusion.