The structure of a small-pore framework sodium stannosilicate Na2SnSi3O9. 2H(2)O (AV-10) has been determined ab initio from powder X-ray diffraction data (XRD). The unit cell is orthorhombic (space group C222(1), Z = 4) with cell dimensions a = 7.9453(5), b = 10.3439(7), c = 11.6252(7) Angstrom, V = 955 Angstrom (3). The structure of AV-10 is composed of corner sharing SnO6 octahedra and SiO4 tetrahedra, forming a three-dimensional framework structure. The SiO4 tetrahedra form helix chains along [001] interconnected by SnO6 octahedra. The SnO6 octahedra are isolated by SiO4 tetrahedra and, thus, there are no Sn-O-Sn linkages. AV-10 has been characterized by chemical analysis, powder XRD, scanning electron microscopy, Si-29, Sn-119, single- and (FAM) triple-quantum Na-23 MAS NMR spectroscopy, thermogravimetry (TGA), and nitrogen adsorption isotherms. The zeolitic water of AV-IO is reversibly lost. The dehydrated material has been studied in situ by powder XRD, TGA, and, in particular, triple-quantum Na-23 MAS NMR.