MgSO4.11H2O (MS11) is the most highly hydrated phase in the binary MgSO4¿H2O system. Whilst lower hydrates in this system have end-member analogues containing alternative divalent metal cations (Ni, Zn, Mn, Cu, Fe, and Co) and exhibit extensive solid solution, we have determined that no other end-member 11-hydrates exist. Our work has revealed that MS11 accepts 20-30 mol. % substitution of either Ni or Zn; at high dopant concentrations, epsomite-structured heptahydrates occur, but at intermediate dopant concentrations we have discovered a new mixed cation hydrate, which we believe to be an undecahydrate. The objectives of this proposal are to characterise the new phase (index and attempt structure solution), to measure the change in MS11 unit-cell parameters with increasing dopant concentration, and to investigate the site preference of dopant atoms in the MS11 structure.