A new niobium(V) silicate, Rb-2(Nb2O4)(Si2O6) center dot H2O, has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and solid-state NMR spectroscopy. It crystallizes in the tetragonal space group P4(3)22 (No. 95) with a = 7.3431(2) angstrom, c = 38.911(3) angstrom, and Z = 8. Its structure contains tetrameric units of the composition Nb4O18, which share corners to form a layer of niobium oxide. The Nb-O layer is a slice of the pyrochlore structure. Neighboring Nb-O layers are linked by vierer single-ring silicates generating eight-ring and six-ring channels running parallel to < 100 > directions, in which the Rb+ cations and water molecules reside. The tantalum analogue was prepared and characterized by powder X-ray diffraction. Upon heating to 500 degrees C, Rb-2(Nb2O4)(Si2O6) center dot H2O loses lattice water molecules, while the framework structure is retained to give the anhydrous compound Rb-2(Nb2O4)(Si2O6), whose structure was also characterized by single-crystal X-ray diffraction. The dehydrated sample absorbs water reversibly, as indicated by powder X-ray diffraction, Rb-2(Nb2O4)(Si2O6) Crystallizes in the tetragonal space group I4(1) (No. 80) with a = 10.2395(6) angstrom, c = 38.832(3) angstrom, and Z = 16.