Top layers of gamma-Al2O3 composite membranes have been modified by the silane coupling technique using phenyltriethoxysilane for improving the separation factor of CO2 to N-2. The separation efficiency of the modified membranes was strongly dependent upon the hydroxylation tendency of the support materials and the amount of the special functional group (i.e. phenyl radical) which was coupled onto a top layer. The separation factor through the TiO2 supported gamma-Al2O3 membrane was found to be fairly enhanced by silane coupling, but in case of the alpha-Al2O3 supported membrane was not. The CO2/N-2 separation factor through the modified gamma-Al2O3/TiO2 composite membrane is 1.7 at 90 degrees C and Delta P = 2 x 10(5) Pa for the binary mixture containing 50 vol% CO2. The separation factor is proportional to the CO2 concentration in the gas mixture, and the modified membrane is stable up to 100 degrees C. The main mechanism of the CO2 transport through the modified gamma-Al2O3 layer is known to be a surface diffusion.