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Graphene oxide membranes show exceptional molecular permeation properties, with promise for many applications 1, 2, 3, 4, 5. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of∼ 9 Å (ref. 4), which is larger than the diameters of hydrated ions of common salts 4, 6. The cutoff is determined by the interlayer spacing (d) of∼ 13.5 Å, typical for graphene oxide laminates that swell in water 2, 4. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here, we describe how to control d by physical confinement and achieve accurate and tunable ion sieving. Membranes with d from∼ 9.8 Å to 6.4 Å are demonstrated, providing a sieve size smaller than the diameters of hydrated ions. In this regime, ion permeation is found to be thermally activated with energy barriers of∼ 10–100 kJ mol–1 depending on d. Importantly, permeation rates …