We synthesized organic-inorganic nanocomposite membranes by decorating the surfaces of mesoporous alumina or silica substrates with a selective organic material that is chemically anchored to the porous surfaces. The mesoporous substrates are either commercially available alumina membranes or custom-synthesized supported ordered mesoporous silica thin films. Linear alkyl molecules or hyperbranched triazine-based dendrimers with nanometer dimensions were covalently grown from the porous membrane surfaces as filling agents to give composite materials. The organic portion of the composite may be modified by varying the length and functionality of the terminating groups and by increasing the number of generations of the dendrimer grown on the surface. The uniformity of the mesoporous substrates and the various controls over chemistry enable the composite membranes to have a constant membrane performance with a high selectivity for target component. The separation focus of this research is the removal of organic molecules from light gas streams. The performance of our composite membranes compares very favorably to the best results reported in the literature for purely polymeric membranes.