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The green crystals of Ni1.5H[Ni(OH)6W6O18]·12.5H2O and blue crystals of Na4[Ni(OH)6W6O18]·16H2O were isolated from the acidified (to Z = ν(H+)/ν(WO) = 1.00) solutions of Ni(NO3)2-Na2WO4-HNO3-H2O systems. The synthesized salts were identified by chemical analysis, XRPA, and IR spectroscopy. It was shown that the heteropolyanions belonged to the Anderson-Evans type of structure. An X-ray diffraction study of Na4[Ni(OH)6W6O18]·16H2O was carried out ( Mr = 1932.07, triclinic, space group P-1, a = 8.0089(11) Å, b = 10.5758(14) Å, c = 12.1987(16) Å; α = 69.268(13)°, β = 71.069(12)°, γ = 83.816(11)°; V = 914.0(2) Å3 at T = 293 K, Z = 1, ρcal = 3.510 g/cm3, F000 = 874, μ = 19.470 mm−1, −16 ≤ h ≤ 16, −21 ≤ k ≤ 21, −24 ≤ l ≤ 24; the final R factors are RF = 0.0277, wR2 = 0.0469 for the observed reflections ( RF = 0.0606, wR2 = 0.0523 for all independent reflections); S = 0.953; CSD-419883). The structure was solved by direct methods and refined in an anisotropic approximation. Hydrogen atoms were found in a difference synthesis and refined in an isotropic approximation with geometrical limitations. The nature of water in Na4[Ni(OH)6W6O18]·16H2O was characterized by DTA. XRPA was used to identify the thermolysis products of heteropoly compounds. [ABSTRACT FROM AUTHOR]