Graphene oxide membranes-partially oxidized, stacked sheets of graphene(1)-can provide ultrathin, high-flux and energy-efficient membranes for precise ionic and molecular sieving in aqueous solution(2-6). These materials have shown potential in a variety of applications, including water desalination and purification(7-9), gas and ion separation(10-13), biosensors(14), proton conductors(15), lithium-based batteries(16) and super-capacitors(17). Unlike the pores of carbon nanotube membranes, which have fixed sizes(18-20), the pores of graphene oxide membranes-that is, the interlayer spacing between graphene oxide sheets (a sheet is a single flake inside the membrane)-are of variable size. Furthermore, it is difficult to reduce the interlayer spacing sufficiently to exclude small ions and to maintain this spacing against the tendency of graphene oxide membranes to swell when immersed in aqueous solution(21-25). These challenges hinder the potential ion filtration applications of graphene oxide membranes. Here we demonstrate cationic control of the interlayer spacing of graphene oxide membranes with angstrom precision using K+, Na+, Ca2+, Li+ or Mg2+ ions. Moreover, membrane spacings controlled by one type of cation can efficiently and selectively exclude other cations that have larger hydrated volumes. First-principles calculations and ultraviolet absorption spectroscopy reveal that the location of the most stable cation adsorption is where oxide groups and aromatic rings coexist. Previous density functional theory computations show that other cations (Fe2+, Co2+, Cu2+, Cd2+, Cr2+ and Pb2+) should have a much stronger cation-pi interaction with the graphene sheet than Na+ has(26), suggesting that other ions could be used to produce a wider range of interlayer spacings.