Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular-scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na−SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K−SO4 show significantly less clustering under equivalent conditions. In mixed systems (NaxK2−xSO4), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure-breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K+ and SO42−, compared with the analogous Na+−SO42− interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na−K−SO4. Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium–sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting-in in ternary hydrothermal solutions of Na−K−SO4 reflects the opposing, but complementary, natures of Na−SO4 versus K−SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.