The synthesis of high-flux composite membranes requires deposition of an ultrathin, discriminating layer on a highly permeable support. This paper describes the synthesis, derivatization, and characterization of hyperbranched poly(acrylic acid) (PAA) membranes on porous alumina supports. PAA films as thin as similar to 40 nm effectively cover underlying pores without filling them, presumably because of their hyperbranched structure. Synthesis of these films begins by sputtering a thin gold layer on the alumina support and then grafting a layer of PAA to a self-assembled monolayer of mercaptoundecanoic acid on the gold. Graft-on-graft deposition of PAA yields the hyperbranched membrane; FESEM (field-emission scanning electron microscopy) and AFM (atomic force microscopy) images clearly show that hyperbranched PAA films can completely cover the substrate surface without filling underlying pores, thus creating an ultrathin membrane on a porous support. PAA membranes are especially attractive because derivatization permits control over transport properties. Gas transport studies indicate that three-layer PAA films do not show a high selectivity by themselves, but selectivity improves significantly after covalent derivatization of PAA with H2NCH2(CF2)(6)CF3.