This study investigates the molecular sieving properties of graphene oxide (GO) membranes, which are vacuum-tight in the dry state but act as molecular sieves when immersed in water. The membranes block solutes with hydrated radii larger than 4.5 Å, allowing smaller ions to permeate with ultrafast rates, many orders of magnitude faster than diffusion alone can account for. The ultrafast ion permeation is attributed to an "ion sponge" effect, where small salts are highly concentrated inside the GO capillaries. The sieving behavior is explained by a network of nanocapillaries that open up in the hydrated state, allowing only species with specific sizes to pass through. The study uses various analytical techniques, including ion chromatography, inductively coupled plasma optical emission spectrometry, and optical absorption spectroscopy, to quantify the permeation rates and identify the solutes that can pass through the membranes. Molecular dynamics simulations support the experimental findings, showing that the capillary-like pressure inside the GO capillaries facilitates the rapid transport of small ions. The GO membranes exhibit extraordinary separation properties, making them promising for filtration and separation technologies.This study investigates the molecular sieving properties of graphene oxide (GO) membranes, which are vacuum-tight in the dry state but act as molecular sieves when immersed in water. The membranes block solutes with hydrated radii larger than 4.5 Å, allowing smaller ions to permeate with ultrafast rates, many orders of magnitude faster than diffusion alone can account for. The ultrafast ion permeation is attributed to an "ion sponge" effect, where small salts are highly concentrated inside the GO capillaries. The sieving behavior is explained by a network of nanocapillaries that open up in the hydrated state, allowing only species with specific sizes to pass through. The study uses various analytical techniques, including ion chromatography, inductively coupled plasma optical emission spectrometry, and optical absorption spectroscopy, to quantify the permeation rates and identify the solutes that can pass through the membranes. Molecular dynamics simulations support the experimental findings, showing that the capillary-like pressure inside the GO capillaries facilitates the rapid transport of small ions. The GO membranes exhibit extraordinary separation properties, making them promising for filtration and separation technologies.