This study reports the unimpeded permeation of water through helium-leak-tight graphene-based membranes. Submicron-thick graphene oxide (GO) membranes are impermeable to gases and liquids but allow rapid water permeation, with water permeating at least 10¹⁰ times faster than helium. This is attributed to the nearly frictionless flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Other molecules are blocked by water that clogs the capillaries and by their reversible narrowing in low humidity. Despite being only one atom thick, graphene is believed to be impermeable to all gases and liquids. However, GO membranes, which are made from a derivative of graphene, are particularly interesting due to their ability to be mass-produced and their mechanical strength and flexibility. The study investigates molecular permeation through such films. The GO membranes were fabricated by exfoliating graphite oxide in water and spin-coating the suspension onto copper foils. The membranes were then etched to create apertures and sealed with copper disks. Permeation experiments showed that GO membranes are nearly impermeable to helium, with an upper limit on permeation rates of approximately 10⁻¹¹ g/cm²·s·bar. In contrast, water permeates through GO membranes at a rate of approximately 10⁻⁵ mm·g/cm²·s·bar, which is ten orders of magnitude faster than helium. The study also investigated the influence of humidity on water permeation through GO membranes. It was found that at 100% relative humidity, the permeation rates of water through GO membranes and open apertures were nearly the same. However, at lower humidity, the permeation rate through GO membranes became much smaller than through open apertures. The blockage induced by low humidity was attributed to changes in the interlayer spacing of GO, which can be reduced to less than 4 Å. The study proposes a model in which GO membranes contain two-dimensional capillaries filled with an ordered monolayer of water. A capillary-like pressure provides a sufficient flow to keep the exposed GO surface wet, limiting the observed permeability by surface evaporation. Permeation of other molecules is blocked by the intercalating water and by their shrinkage in low humidity. The results have implications for the use of graphene oxide in various applications, explaining why the observed surface areas are close to the theoretical maximum. The study also suggests that the phenomenon can be utilized in filtration and separation technologies, possibly along the lines of those extensively discussed for membranes made from carbon nanotubes.This study reports the unimpeded permeation of water through helium-leak-tight graphene-based membranes. Submicron-thick graphene oxide (GO) membranes are impermeable to gases and liquids but allow rapid water permeation, with water permeating at least 10¹⁰ times faster than helium. This is attributed to the nearly frictionless flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Other molecules are blocked by water that clogs the capillaries and by their reversible narrowing in low humidity. Despite being only one atom thick, graphene is believed to be impermeable to all gases and liquids. However, GO membranes, which are made from a derivative of graphene, are particularly interesting due to their ability to be mass-produced and their mechanical strength and flexibility. The study investigates molecular permeation through such films. The GO membranes were fabricated by exfoliating graphite oxide in water and spin-coating the suspension onto copper foils. The membranes were then etched to create apertures and sealed with copper disks. Permeation experiments showed that GO membranes are nearly impermeable to helium, with an upper limit on permeation rates of approximately 10⁻¹¹ g/cm²·s·bar. In contrast, water permeates through GO membranes at a rate of approximately 10⁻⁵ mm·g/cm²·s·bar, which is ten orders of magnitude faster than helium. The study also investigated the influence of humidity on water permeation through GO membranes. It was found that at 100% relative humidity, the permeation rates of water through GO membranes and open apertures were nearly the same. However, at lower humidity, the permeation rate through GO membranes became much smaller than through open apertures. The blockage induced by low humidity was attributed to changes in the interlayer spacing of GO, which can be reduced to less than 4 Å. The study proposes a model in which GO membranes contain two-dimensional capillaries filled with an ordered monolayer of water. A capillary-like pressure provides a sufficient flow to keep the exposed GO surface wet, limiting the observed permeability by surface evaporation. Permeation of other molecules is blocked by the intercalating water and by their shrinkage in low humidity. The results have implications for the use of graphene oxide in various applications, explaining why the observed surface areas are close to the theoretical maximum. The study also suggests that the phenomenon can be utilized in filtration and separation technologies, possibly along the lines of those extensively discussed for membranes made from carbon nanotubes.