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. However, our work has revealed that MS11 grown slowly from aqueous solution accepts up to 50 mol. % substitution of either Co or Mn. The objective of this proposal is to characterize the distribution of cation substituents, identifying any preference for either of the two symmetry inequivalent Mg sites in the MS11 structure. We will also characterize any changes in the hydrogen bond network (and the orientation of metal-coordinated waters) caused by changes in the size and/or distortion of the M(H2O)6 octahedra.