The binary and ternary aqueous solutions of sodium phosphate or/and sulfate were studied using the hygrometric method from dilution to saturation in the temperature range from (298.15 to 353.15) K. The hygrometric measurements of the binary systems were combined with reference pure water, Na 3 PO 4 (s), and Na 2 SO 4 (s) solubility data to construct a chemical model that calculates solute and solvent activities from dilute to saturated solution concentration (for (Na 3 PO 4 (s)) up to m max = 9.00 mol·kg −1 at 353.15 K; and up to m max (Na 2 SO 4 (s)) = 3.40 mol·kg −1 at 313.15 K). The constructed model was also used to calculate thermodynamic solubility product (K°s p ) of Na 3 PO 4 (s) and Na 2 SO 4 (s) in the temperature range (298.15 to 353.15) K. The excess Gibbs energies of Na 3 PO 4 (aq) and Na 2 SO 4 (aq) were evaluated at various temperatures. The results for these aqueous solutions were influenced by the effect of hydration shell, with ion association or formation ion pairing. The anion solute PO 4 or SO 4 influences the hydration in an aqueous solution; indeed the hydrogen bonding of water in the hydration shells of PO 4 3− anion is preeminent over that of SO 4 2− . The mixed electrolyte solutions {(yNa 3 PO 4 + (1 − y)Na 2 SO 4 }(aq) were also investigated in the temperature range from (298.15 to 353.15) K and at total molalities from dilution (0.10 mol·kg −1 ) to saturation for different ionic-strength fractions (y) of Na 3 PO 4 with y = 0.20, 0.50, and 0.80. The water activity of mixed salts solutions and related properties such as osmotic and activity coefficients were carried out at various temperatures. The osmotic coefficients data were used to evaluate the mixing parameters of the ion interaction model θ MN and ψ MNX in the ternary system Na 3 PO 4 + Na 2 SO 4 + H 2 O.