A sodium titanosilicate of ideal composition Na2TÍ203Si04'2H20 was synthesized hydro-thermally in highly alkaline media. Its crystal structure was solved from X-ray powder data by ab initio methods. The compound is tetragonal, a=

7.8082 (2), c= 11.9735 (4) Á, space group PA^ Jmcm, Z— A. The titanium atoms occur in clusters of four grouped about the 42 axis and are octahedrally coordinated by oxygen atoms. The silicate groups serve to link the titanium clusters intogroups of four arranged in a square of about 7.8 Á in length. These squares are linked to similar ones in the c direction by sharing corners to form a framework which encloses a tunnel. Half the Na+ ions are situated in the framework coordinated by silicate oxygen atoms and water molecules. The remaining sodiums are present in the cavity, but evidence indicates that some of them are replaced by protons. The composition is then closer to Nai. eiHo. se^ OsSiCUfi. S^ O. The sodium ions within the tunnels are exchangeable. Exhaustive exchange with CsNOs yielded a Cs+ phase of composition NaiAgCso^ Ho. si^ OsSiCh-^ O whose structure was revealed from application of Rietveld methods to X-ray powder data.