We have recently reported the fabrication of metal–organic framework (MOF) membranes for potential application for selective removal of ions from water. Ultrathin and defect-free MOF membranes are required to achieve high ion flux. We have developed new seeding strategies for fabricating ultrathin and high-quality molecular sieving membranes on various porous substrates. For instance, hybrid MOF nanosheets are formed by growing MOF nanocrystals on both sides of 2D graphene oxide nanosheets. These flexible, micro-sized hybrid nanosheets allow for seeding substrates with large-pores and rough surface, eliminating substrate modification required in the conventional synthesis using nanocrystal seeds. We have further developed a novel nanocrystal-mask plasma etching method for synthesizing MOF/mesoporous GO (MOF/MGO) hybrid nanosheets. And then, a MOF membrane was fabricated by intergrowth of MOF within a preformed laminated MOF/MGO matrix. Due to their ultrathin thickness and well-aligned MGO nanosheets, this hierarchical structural MOF/MGO membrane is suitable for studying ion transport and selection. We have found that such membranes have fast, selective ion permeation properties and demonstrated selective transport of various cations and anions in MOF-based membranes.