A composite ceramic-organic membrane has been prepared by chemical grafting of organo-phosphate molecules to the surface of an aluminium-oxide membrane. Gas-transport mechanism through the initial mesoporous membrane with pore size of 5 nm is essentially based on Knudsen diffusion and so does not give significant separation factors between gases of similar molecular weights. Modification of membrane surface properties allows control of the relative contribution of differing transport mechanisms. Modified membranes have been tested for various gas permeations (methane, ethane, propane, hydrogen, nitrogen and carbon dioxide) at room temperature. The modified membranes display high permeability and high selectivity coefficient for propane/nitrogen separation. The chemical, physical and geometrical properties of the modifying molecules can be chosen in order to improve the performances of any specific application.