In this investigation, the thermodynamic properties of the aqueous solutions of orthophosphate salts M3-nHnPO4 H2O with n=0, 1, or 2 and M =Na+, K+ or NH4 at 298.15 K are determined. The water activity measurements for NaH2PO4(aq), Na2HPO4(aq), Na3PO4(aq); KH2PO4(aq), K2HPO4(aq), K3PO4(aq), (NH4)H2PO4(aq), (NH4)HPO4(aq) and (NH4)(3)PO4(aq) are effectuated from dilution to saturated solution using the hygrometric method. The activity coefficients are evaluated using ionic-interaction model. The mineral solubility of these phosphate salts is investigated at T = 298.15 K. The prediction of solubility is made for minerals and other solid phases in aqueous solutions of phosphates. Correlating and predicting the phase behavior in such systems is a difficult task, partially caused by chemical reactions in the liquid phase but also due to the possible formation of one or more solid phases. Solid phases, molality of saturated binary solutions m(s(exp)) and logarithm of solubility product K-sp are also determined at 298.15K. The standard molar Gibbs energies of dissolution Delta G (diss), formation Delta G(f Cals), and the literature Delta G/(f Ref), of solid phases crystallizing from saturated phosphate salts solutions are evaluated. This investigation allows understanding physicochemical nature and structure of phosphate solutions, in particular, questions of anion or cation hydration, especially at higher concentrations, ion-pairing formation, and the possibility of phosphate complex formation. (c) 2014 Elsevier B.V. All rights reserved.