Gas separation composite membranes have been prepared using a porous substrate and a thin organic-inorganic sol-gel layer as a permselective coating. These permselective layers have been obtained by co-hydrolysis of an organoalkoxisilane (C6H5)(n)-Si(OCH3)(4-n) [n = 1, 2] and tetramethoxysilane Si(OCH3)(4). Various molecular structures are formed depending on the functionality of the organoalkoxide and its proportion in the mixture. These networks have been characterized by solid-state NMR, small-angle X-ray scattering and differential scanning calorimetry. Single gas flux measurements for various gases (CO2, N-2, He) have been obtained and ideal CO2/N-2, He/N-2 selectivities calculated. Gas permeation results are discussed in terms of structural characterization.