The osmotic coefficients of aqueous mixtures of KNO3 and K2HPO4 have been measured at T = (298.15 +/- 0.01) K by the isopiestic vapor pressure method at KNO3 ionic strength fractions y = (0.20581, 0.43631, 0.61099, and 0.83170), over the ionic strength range (2.4958 to 6.0801) mol.kg (1), using KCl(aq) and CaCl2(aq) as the reference standard solutions. These measurements include some new isopiestic results for the KNO3(aq) binary system extending slightly into the oversaturated (supersaturated) molality region from (2.1973 to 4.1295) mol.kg (1). Our new experimental results were modeled with an extended form of Pitzer's ion-interaction model equations, both with the usual Pitzer mixing terms and with Scatchard's neutral-electrolyte mixing terms, and with the Clegg-Pitzer-Brimblecombe equations based on the mole-fraction-composition scale. There are no previously published isopiestic measurements for the {yKNO(3) + (1 - y)K2HPO4}(aq) system. The present assessment yields NO3 - HPO42 mixing parameters for these models that are needed for modeling the thermodynamic activities of solute components of natural waters and other complex aqueous electrolyte mixtures. Model parameters for KNO3(aq) at T = (298.15 +/- 0.01) K were also evaluated using the present results along with critically-assessed osmotic coefficients taken from the published literature. Thermodynamic properties of the saturated solution were calculated using published solubilities of KNO3(cr) in water at T = 298.15 K and the evaluated ion-interaction model parameters for KNO3(aq). (C) 2012 Elsevier Ltd. All rights reserved.