The proton form of the exchanger Na2Ti2O3(SiO4). 2H(2)O exhibits a high selectivity for Cs+ even in acid solutions. A neutron diffraction study was carried out to locate the position of the exchangeable protons. The structure contains cubane-like Ti4O4 clusters bridged by silicate groups. The acidic proton is located on the cubane oxygens which make them four-coordinate, being bonded to three titaniums and one proton. The unit cell is tetragonal, a = 11.0343(5) Angstrom, c = 11.8797(7) Angstrom, space group P4(2)/mbc, Z = 8. There are six water molecules per tunnel and two tunnels per unit cell. Thus, the empirical formula is Ti-2(OH)(2)OSiO4. 1.5H(2)O. The water molecules are H-bonded to the framework oxygens and to each other. Although the tunnels are filled with water molecules, framework sites normally occupied by Na+ are empty. The high selectivity for Cs+ apparently stems from the ability to form a thermodynamically stable eight-coordinate complex by replacement of the hydroxyl protons.