At ambient temperature, liquid water transforms from a low-density to a high-density dynamic structure at ~0.2 GPa. The transition persists in electrolyte solutions; however, its effects on solute properties are unknown. We obtained Raman spectra of 0.5–2.0 molal MgSO4 solutions at 21 °C and 10-4 to ~1.6 GPa. Above about 0.4 GPa, we observed an increase in the MgSO4 contact ion pair abundance with pressure, regardless of concentration. This phenomenon contravenes the general rule that dissolved salts dissociate upon compression, and is likely caused by the structural collapse in the solvent with pressure due to increased hydrogen-bond breaking. Increasing ion association in high-pressure aqueous solutions implies that, at a given salinity, high-density water in deep, cold planetary oceans and pore waters will possess lower ionic strength and electric conductivity than previously thought. This behaviour will also lead to higher ocean salinity in the interiors of Pluto and the largest icy moons of Jupiter and Saturn, Ganymede, Callisto, and Titan, or in exoplanet water-worlds, through enhancement of submarine silicate weathering.