Mg-sulfate 11-hydrate (meridianiite) is decomposed by pressure to form Mg-sulfate 9-hydrate + ice, a process that will occur in the mantles of large icy planetary bodies and have the same impact as olivine's decomposition to Mg-silicate perovskite + MgO in the Earth's mantle.It is necessary for accurate modelling of planetary interiors to determine the high-pressure behaviour of Mg-sulfate 9-hydrate, which includes measurement of its incompressibility (K), as well as the pressure and temperature dependence of K, and characterisation of any phase changes. Some sparse and low-quality data already obtained, combined with DFT calculations, indicate a bulk elastic softening above 1 GPa that may indicate a further phase transition or dehydration. I propose to carry out a gas-cell study on OSIRIS (0-550 MPa, 100-240 K) and a Paris-Edinburgh cell study on PEARL (0-5 GPa, 100-380 K).