SeO₂ and As₂O₃ were reacted at 200 °C in in a potassium hydroxide hydroflux with a molar water-base ratio n(H₂O) : n(KOH)=1.8. Arsenate(III) disproportionates under these conditions into arsenate(V) and elemental arsenic. The reduction also yielded tetraselenodiarsenate(II) anions, which turned the solution yellow and precipitated at room temperature as plate-shaped crystals of K₄(As₂Se₄) ⋅ 2H₂O. The unexpected redox reaction as well as the formation of a hygroscopic compound in an aqueous solution are attributed to a strongly reduced activity of water in ultra-alkaline media. K₄(As₂Se₄) ⋅ 2H₂O crystallizes in the orthorhombic space group Cmce with lattice parameters a=784.78(3), b=1027.76(4) and c=1750.63(7) pm at 100 K. The (As₂Se₄)⁴⁻ anion has C_2h symmetry and an As−As bond length of 245.3(1) pm. The water molecules are part of infinite zig-zag chains (Formula presented.) [K(H₂O)_2/2]⁺ and form hydrogen bonds to neighboring selenium atoms.