Graphene oxide (GO) membranes are used for ion sieving and desalination, but their effectiveness is limited by a permeation cutoff of ~9 Å, which is larger than the hydrated diameters of common ions. This cutoff is determined by the interlayer spacing (d) of the GO membranes, which typically swells in water to ~13.5 Å. By physically confining GO membranes, researchers have demonstrated the ability to tune d from ~9.8 Å to 6.4 Å, enabling ion sieving smaller than typical hydrated ion diameters. Ion permeation through these membranes is thermally activated, with energy barriers of ~10-100 kJ/mol depending on d. Permeation rates decrease exponentially with smaller d, while water transport is weakly affected. This is attributed to a low barrier for water entry and large slip lengths in graphene capillaries. The study also shows that GO-based membranes with limited swelling can achieve 97% rejection for NaCl. The findings suggest that controlling d in GO membranes can enhance ion selectivity, which is crucial for desalination and water filtration. The research highlights the potential of GO membranes for applications requiring precise ion sieving, with the ability to tune d through physical confinement. The membranes were fabricated by controlling humidity and using epoxy to restrict swelling. The results demonstrate that GO membranes with smaller d can effectively sieve ions, while water transport remains efficient. The study also shows that the energy barrier for ion permeation is significantly higher for divalent ions compared to monovalent ones, indicating that dehydration plays a key role in ion selectivity. The findings provide a pathway for developing GO membranes with tunable interlayer spacing for desalination applications.Graphene oxide (GO) membranes are used for ion sieving and desalination, but their effectiveness is limited by a permeation cutoff of ~9 Å, which is larger than the hydrated diameters of common ions. This cutoff is determined by the interlayer spacing (d) of the GO membranes, which typically swells in water to ~13.5 Å. By physically confining GO membranes, researchers have demonstrated the ability to tune d from ~9.8 Å to 6.4 Å, enabling ion sieving smaller than typical hydrated ion diameters. Ion permeation through these membranes is thermally activated, with energy barriers of ~10-100 kJ/mol depending on d. Permeation rates decrease exponentially with smaller d, while water transport is weakly affected. This is attributed to a low barrier for water entry and large slip lengths in graphene capillaries. The study also shows that GO-based membranes with limited swelling can achieve 97% rejection for NaCl. The findings suggest that controlling d in GO membranes can enhance ion selectivity, which is crucial for desalination and water filtration. The research highlights the potential of GO membranes for applications requiring precise ion sieving, with the ability to tune d through physical confinement. The membranes were fabricated by controlling humidity and using epoxy to restrict swelling. The results demonstrate that GO membranes with smaller d can effectively sieve ions, while water transport remains efficient. The study also shows that the energy barrier for ion permeation is significantly higher for divalent ions compared to monovalent ones, indicating that dehydration plays a key role in ion selectivity. The findings provide a pathway for developing GO membranes with tunable interlayer spacing for desalination applications.