A zirconium trisilicate compound, with composition K2ZrSi3O9 . H2O (1), was prepared under mild hydrothermal conditions and was structurally characterized by using its X-ray powder diffraction data. The compound crystallizes in the space group P2(1)2(1)2(1) With a 10.2977(2) Angstrom, b = 13.3207(3) Angstrom, c = 7.1956(1) Angstrom, and Z = 4. The asymmetric unit consists of a metal atom, a trisilicate group, and three lattice positions corresponding to two cations and a water oxygen atom. In the structure, the Zr atom is octahedrally coordinated by the six terminal oxygens of the trisilicate group. The trisilicate groups exist as linear chain polymers connected to each other through the Zr atoms. This arrangement leads to channels and cavities in the structure that are occupied by the cations and water molecules. The K+ ions in compound 1 were exchanged for Cs+ ions in two steps. Ln the first case about 50% of the K+ ions were exchanged to give a compound with composition K0.9Cs1.1ZrSi3O9 . H2O (2). Compound 2 was then loaded with additional Cs+ ions which resulted in a phase K0.5Cs1.5ZrSi3O9 . H2O (3). These exchanged phases retain the crystal symmetry of compound 1, but their unit cell dimensions have expanded as a result of large Cs+ ions replacing the smaller K+ ions. Structure analyses of the exchanged phases show that the cations found in the cavities of compound 1 are highly selective for Cs+ ions. A small amount of Cs ions also go to a site in the large channel that is very close to that occupied by the water oxygen in compound 1. In the absence of Cs, this site is filled with water molecules. The second cation found in the channel of 1 is partially occupied by water and K+ ions. The K+ ions in compound 1 were completely exchanged for Naf ions, and the compound thus obtained, Na2ZrSi3O9 . H2O (4), was treated with Cs+ ions in a manner similar to that carried out for compound 1. The low Csf ion phase, Na0.98Cs1.02ZrSi3O9 . H2O (5), and the high Cs+ ion phase, Na0.6Cs1.4-ZrSi3O9 . H2O (6), show ion distributions very similar to compounds 2 and 3 except for the fact that in the Na phases a small amount Csf ion also goes to the second cation site. Compound 1 on heating releases the lattice water and transforms into a hexagonal phase, K2ZrSi3O9, corresponding to the mineral wadeite. In the high-temperature phase the silicate group exists as a condensed cyclic group and the K+ ions are sandwiched between trisilicate groups. A possible pathway for this conversion is also discussed.