Graphene oxide (GO) membranes exhibit precise and ultrafast molecular sieving properties. These membranes are vacuum-tight in the dry state but become molecular sieves when immersed in water, blocking solutes with hydrated radii larger than 4.5 Å. Smaller ions permeate through the membranes with minimal resistance, much faster than expected by diffusion mechanisms. This behavior is attributed to a network of nanocapillaries that open in the hydrated state, allowing only species that fit through. The ultrafast separation of small salts is due to an "ion sponge" effect, where highly concentrated salt solutions form inside the graphene capillaries. GO membranes have a narrow pore size distribution in the angstrom range (1-5 Å), making them suitable for filtration and separation technologies. They are easy to fabricate, mechanically robust, and suitable for industrial-scale production. GO membranes are made of impermeable functionalized graphene sheets with a typical size of 1 µm and sufficient interlayer separation to accommodate a mobile layer of water. GO membranes were prepared by vacuum filtration from GO suspensions. They were tested for continuity using a helium leak detector, confirming their vacuum-tightness in the dry state. When filled with water solutions, the membranes allowed rapid permeation, driven by osmosis. Water flow rates were found to be approximately 0.2 L m⁻² h⁻¹ for 1 M feed solutions, increasing with molar concentration. Ion permeation through GO membranes was studied using various analytical techniques. Small ions like Mg²⁺ and Cl⁻ permeated through the membranes, while larger ions and organic molecules did not. The permeation rates were found to be linear with the feed concentration, and the observed rates depended on the hydrated radius of the ions. The effective pore size was determined to be approximately 9 Å, consistent with the mesh size found experimentally. Molecular dynamics simulations supported the experimental findings, showing that ions enter the capillaries and diffuse into the permeate reservoir. The permeation rates were found to be similar for all small ions and showed little dependence on ionic charge. Larger species like toluene and octanol could not permeate even through capillaries containing three monolayers of water. The ion sponge effect was confirmed by measuring the salt intake by GO membranes, which reached up to 25% of the membrane's initial weight. This indicates highly concentrated solutions inside the graphene capillaries. The energy gain from ions moving inside the capillaries from the bulk solution was confirmed by simulations, indicating that the capillary-like pressure acts on ions rather than water molecules. GO membranes exhibit extraordinary separation properties, and further experimental and theoretical work is needed to fully understand their behavior. The functionalized regions of GO membranes may play a role in enhancing the ion sponge effect. GO membranes with ultrafast ion transport and atomic-scale pores are promising materials for separation and filtration technologies, particularly for extracting valuable solutes from complexGraphene oxide (GO) membranes exhibit precise and ultrafast molecular sieving properties. These membranes are vacuum-tight in the dry state but become molecular sieves when immersed in water, blocking solutes with hydrated radii larger than 4.5 Å. Smaller ions permeate through the membranes with minimal resistance, much faster than expected by diffusion mechanisms. This behavior is attributed to a network of nanocapillaries that open in the hydrated state, allowing only species that fit through. The ultrafast separation of small salts is due to an "ion sponge" effect, where highly concentrated salt solutions form inside the graphene capillaries. GO membranes have a narrow pore size distribution in the angstrom range (1-5 Å), making them suitable for filtration and separation technologies. They are easy to fabricate, mechanically robust, and suitable for industrial-scale production. GO membranes are made of impermeable functionalized graphene sheets with a typical size of 1 µm and sufficient interlayer separation to accommodate a mobile layer of water. GO membranes were prepared by vacuum filtration from GO suspensions. They were tested for continuity using a helium leak detector, confirming their vacuum-tightness in the dry state. When filled with water solutions, the membranes allowed rapid permeation, driven by osmosis. Water flow rates were found to be approximately 0.2 L m⁻² h⁻¹ for 1 M feed solutions, increasing with molar concentration. Ion permeation through GO membranes was studied using various analytical techniques. Small ions like Mg²⁺ and Cl⁻ permeated through the membranes, while larger ions and organic molecules did not. The permeation rates were found to be linear with the feed concentration, and the observed rates depended on the hydrated radius of the ions. The effective pore size was determined to be approximately 9 Å, consistent with the mesh size found experimentally. Molecular dynamics simulations supported the experimental findings, showing that ions enter the capillaries and diffuse into the permeate reservoir. The permeation rates were found to be similar for all small ions and showed little dependence on ionic charge. Larger species like toluene and octanol could not permeate even through capillaries containing three monolayers of water. The ion sponge effect was confirmed by measuring the salt intake by GO membranes, which reached up to 25% of the membrane's initial weight. This indicates highly concentrated solutions inside the graphene capillaries. The energy gain from ions moving inside the capillaries from the bulk solution was confirmed by simulations, indicating that the capillary-like pressure acts on ions rather than water molecules. GO membranes exhibit extraordinary separation properties, and further experimental and theoretical work is needed to fully understand their behavior. The functionalized regions of GO membranes may play a role in enhancing the ion sponge effect. GO membranes with ultrafast ion transport and atomic-scale pores are promising materials for separation and filtration technologies, particularly for extracting valuable solutes from complex