[en] For the first time single crystals of Na₄WO₅ were prepared by heating of mixtures of Na₂WO₄ and Na₂O [Na:W = 4.2:1, Ni-cylinder, 725 ⁰C, 28 d). Four-circle-diffractometer data [PW 1100, 1900 I_o(hkl), ω-2Θ scan, AgKα-bar-radiation, R = 6.5%, R_w = 5.7%]determine the triclinic structure of Na₄WO₅ with a = 569.4(2), b = 847.7(2), c = 565.7(1) pm, α = 101.29(2)⁰, β = 102.25(3)⁰, γ = 109.20(3)⁰. It is isotypic with the NaCl-type of structure Li₄WO₅. By heating Na₄WO₅ with K₂O [Na:K:W = 4:3:1, Au-tube, 750 ⁰C, 35 d] colourless single crystals of KNa₃WO₅ were obtained for the first time. The structure determination [four-circle-diffractometer data, PW 1100, ω-2Θ scan, 943 I₀(hkl), MoKα-bar-Strahlung R = 4.9%, R_w = 3.9%] proves space group C2/m with a = 907.8(3), b = 753.9(2), c = 911.8(3) pm, β = 104.07(4)⁰, Z = 4. The structure is characterized by undulating layers of Na, W and O and two different kinds of channels. The Madelung Part of Lattice Energy, MAPLE, Effective Coordination Numbers, ECON, these via Mean Fictive Ionic Radii, MEFIR, are calculated and discussed. (author)