The crystal structures of bis(2-thiobarbiturato-O)tetraaquamagnesium Mg(H2O)4(HTBA-O)2 I and catena-[(μ2-2-thiobarbiturato-O,O)(2-thiobarbiturato-O)bis(μ2-aqua)diaquastrontium] monohydrate catena-[Sr(μ2-H2O)2(H2O)2(μ2-HTBA-O,O)(HBTA-O)] n · nH2O (II), where H2TBA is 2-thiobarbituric acid C4H4N2O2S, have been determined. Crystal data for a=6.7598(2) Å, b = 7.6060(2) Å, c = 8.5797(2) Å, α = 79.822(2)°, β = 76.622(1)°, γ = 69.124(1)°, V = 398.82(2) Å3, space group P , Z = 1; for II: a = 20.8499(4) Å, b = 19.2649(5) Å, c = 4.14007(9) Å, β = 92.023(2)°, V = 1661.91(7) Å3, space group P21/n, Z = 4. The Mg2+ ion in I is bonded to six O atoms of two HTBA− ions and four water molecules that form a nearly regular octahedron. Each Sr2+ ion in II is coordinated to three oxygen atoms of three HTBA− ions and six water molecules that form an almost ideal tricapped trigonal prism. These polyhedra share edges to form infinite chains. Intermolecular hydrogen bonds create layered structures of I and II.