In this investigation, the thermodynamic properties of aqueous solutions Na, K or NH4 dihydrogenphosphate were determined at various temperatures ranging from 298.15 K to 353.15 K. The measurements of water activity were performed in the molality range from dilution to saturation using the hygrometric method. The modeling approach based on the ion interaction model was developed to determine the thermodynamic properties. From these measurements of phosphate salts, mixing parameters in binary solutions were evaluated at various temperatures, without considering formation of aqueous complexes. The osmotic and activity coefficients were determined for these aqueous solutions of orthophosphate salts. The measurements of the binary systems were combined with reference pure water, NaH2PO4(s), KH2PO4(s) and (NH4)H2PO4(s) solubility data to construct a chemical model that calculates solute and solvent activities from dilute to saturated solution concentration from 298.15 to 353.15 K respectively up to m(max)(NaH2PO4(s)) = 7.90-18.29 mol kg(-1); m(max)(KH2PO4(s)) = 1.83 -5.10 mol kg(-1); and m(max)((NH4)H2PO4(s)) = 4.93-12.62 mol kg(-1). The thermodynamic characteristics, solubility products K-sp degrees and the standard molar Gibbs energy of dissolution Delta G(diss)degrees were also given. (C) 2015 Elsevier B.V. All rights reserved.