Abstract : Salts and volatiles could have a major impact on the structure of extraterrestrial hydrospheres. Main consequences include a greater stability of aqueous fluids (favoring the stability of subsurface oceans) and the formation of new solids (affecting the physical properties of icy layers). However, most aqueous systems of interest remain poorly known at the high pressures (GPa) expected inside large water-rich worlds. Dedicated high-pressure experiments may provide the data required to feed planetary evolution models. This presentation will illustrate the use of optical anvil cells to constrain phase equilibria (example of the H2O-CO2 system), and the use of acoustic measurements to establish the equation of state of fluid samples (example of Na-Mg-Cl-SO4 brines).