Two new quaternary sodium tungsten selenites, Na-2(WO3)(3)(SeO3)center dot 2H(2)O (P3(1)c) and Na-6(W6O19)(SeO3)(2) (C2), have been synthesized and characterized. The former exhibits a hexagonal tungsten oxide layered structure, whereas the latter has a one-dimensional "ribbon" structure. The layers and "ribbons" consist of distorted WO6 and asymmetric SeO3 polyhedra. The layers in Na-2(WO3)(3)(SeO3)center dot 2H(2)O and the "ribbons" in Na-6(W6O19)(SeO3)(2) are separated by Na+ cations. Powder second-harmonic-generation (SHG) measurements on Na-2(WO3)(3)(SeO3)center dot 2H(2)O and Na-6(W6O19)(SeO3)(2) using 1064 nm radiation reveal SHG efficiencies of approximately 450X and 20X alpha-SiO2, respectively. Particle size versus SHG efficiency measurements indicate that the materials are type 1 non-phase-matchable. Converse piezoelectric measurements result in d(33) values of approximately 23 and 12 pm/V, whereas pyroelectric measurements reveal coefficients of -0.41 and -1.10 mu C/m(2).K at 60 degrees C for Na-2(WO3)(3)(SeO3)center dot 2H(2)O and Na-6(W6O19)(SeO3)(2), respectively. Frequency-dependent polarization measurements confirm that the materials are nonferroelectric; i.e., the macroscopic polarization is not reversible, or "switchable". IR and UV vis spectroscopy, thermogravimetric and differential thermal analysis measurements, and electron localization function calculations were also done for the materials. Crystal data: Na-2(WO3)(3)(SeO3)center dot 2H(2)O, trigonal, space group P3(1)c (No. 159), a = 7.2595(6) angstrom, b = 7.2595(6) angstrom, c = 12.4867(13) angstrom, V = 569.89(9) angstrom(3), Z = 2; Na-6(W6O19)(SeO3)(2), monoclinic, space group C2 (No. 5), a = 42.169(8) angstrom, b = 7.2690(15) angstrom, c = 6.7494(13) angstrom, beta = 98.48(3), V = 2046.2(7) angstrom(3), Z = 4.