A comprehensive set of interaction parameters for the thermodynamic modelling of PO43 , HPO42 and H2PO4 in concentrated solutions in the Na-K-Cl-SO4-H2O system at 25 degrees C was calculated by regression of experimental isopiestic and solubility data with the Pitzer formalism. The isopiestic method was used to generate additional sets of activity data, which complete and improve the statistical quality of data extracted from an extensive literature review. Thermodynamic properties of phosphate phases were calculated. In this first part, binary systems are presented. (C) 2013 Elsevier Ltd. All rights reserved.