We report atomic layer deposition of an ultrathin hybrid organic/inorganic film on a porous support and its conversion to a high flux, high selectivity membrane. Through chemical passivation of the internal support porosity and remote oxygen plasma activation of the support surface, ALD of a bis(triethoxysilyl)ethane precursor is confined to the immediate surface of the support, allowing formation of a 5 nm thick film spanning the underlying porosity. UV/ozone removal of the C-2 porogen creates a microporous membrane with a He/SF6 selectivity > 10(4) and a substantial He flux of 5.3 sccm/bar.cm(2). Prior to conversion, these ultrathin films are of interest as low k dielectric sealing layers. Use of bridging ligands with other shapes and sizes should enable generalization of this approach to other demanding separation problems.