[en] A framework potassium titanium silicate K₂TiSi₃O₉·H₂O, compound I, was synthesized by the reaction of a titanium-hydrogen peroxide complex and SiO₃ in alkaline media under mild hydrothermal conditions (180 C). This compound was converted to the corresponding sodium phase, Na₂TiSi₃O₉·H₂O (IV) and two proton-containing phases, K_1.26H_0.74TiSi₃O₉·1.8H₂O (II) and K_0.3H_1.7TiSi₃O₉·2.4H₂O (III) by ion exchange. These products were characterized by elemental analysis, TGA, FT-IR, MAS ²⁹Si NMR, and X-ray diffraction. The ion exchange behavior of compound I and III toward alkali, alkaline earth, and some transition metal ions solutions was studied. A high affinity of the protonic form of titanium trisilicate exchanger for cesium and potassium makes it promising for radionuclide-contaminated groundwater treatment and certain analytical separations. The crystal structure of compound I was found to be isomorphous with that of the zirconium analogue and contains a framework enclosing two types of tunnels. The exchange properties were interpreted on the basis of this structure and selectivity of the Zr and Ti phases rationalized on the basis of the tunnel sizes. The structure of II was solved on the basis of a monoclinic cell, whereas the compound I phase is orthorhombic. The relationship of structure II, monoclinic, to the parent orthorhombic structure is described. Phase III yielded a complex X-ray pattern with evidence of disorder and a highly complex ²⁹Si NMR spectrum. On reexchanging with K⁺, the original crystal lattice was restored