The crystal structure of K2[5-CO2-2-TU] · 1.5 H2O, C5H5K2N2O4.5S, has been determined at room temperature. The colorless crystals are monoclinic, space group C2/c with unit cell dimensions a = 14.238(6) Å, b = 6.893(4) Å, c = 20.050(4) Å, β = 102.34(2)°, Z = 8 and Dx = 1.903 Mg m−3. The structure was solved by direct methods and refined by a full-matrix least-squares procedure to final R = 0.029 using 1494 reflections. The structure is comprised of K+ cations, 5-carboxylate-2-thiouracilate dianions and water molecules in the ratio 4:2:3. The analysis shows that deprotonation of the thionucleobase occurs at the thioamide N(1) atom and at the carboxyl group. The two cations exist in seven coordinate NO5S geometries, one based on a pentagonal bipyramid and the other based on a capped octahedron.