The coordination of copper(II) by the acyclic polydentate ligand ethylene-1,2-bisoxamate [(oeo)^4–] results in the formation of an anionic dinuclear double-stranded helicate, [Cu₂(oeo)₂]^4–. The potassium salt of this large, chiral, approximately spherical, D₂ symmetric and highly charged complex molecular anion is found to crystallise in five different hydrated forms; a trihydrate 6a, a tetrahydrate 6b, a tetrahemihydrate 6c, a pentahydrate 6d and an approximate heptahydrate 6e. All of these forms crystallise from solution under similar conditions. Together this large anion and the relatively small K⁺ cation provide something of a shape-awkward system, as evidenced by the high degree of pseudo-polymorphism. X-ray structural analysis of 6a–e reveals much similarity in the order, symmetry and packing of the anions, but great irregularity and disorder in the packing of the K⁺ counterions and water of crystallisation. The structures are further complicated by a Cu nonstoichiometry. Although 6e has a 100 % Cu site occupancy, 6a and 6c have site occupancies less than 100 % and contain both [Cu₂(oeo)₂]^4– and [Cu(Hoeo)₂]^4– complex ions as a solid solution. In 6b and 6d, where the Cu site occupancy is below 50 %, the compound comprises [Cu(Hoeo)₂]^4– and [(H₂oeo)₂]^4– units distributed over the same site. It is a remarkable feature of this system that hydrogen bond interactions between pairs of the protonated acyclic ligands result in a dimeric structure, which has the same shape, charge and structure as that of the copper complex of the fully deprotonated ligand. So similar are the [Cu₂(oeo)₂]^4–, [Cu(Hoeo)₂]^4– and [(H₂oeo)₂]^4– components that they are undifferentiated in the crystal structures, and the solid solutions that they form can be viewed as molecular alloys.