The osmotic coefficients of aqueous mixtures of KCl and K2HPO4 have been measured at T = (298.15 +/- 0.01) K by the isopiestic vapor pressure method over the range of ionic strengths from (2.3700 to 11.250) mol . kg(-1) using CaCl2(aq) as the reference solution. Our new experimental results were modeled with an extended form of Pitzer's ion-interaction model equations, both with the usual 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 is a dearth of previously published isopiestic data for mixtures containing salts of HPO42- (aq) and, consequently, no previous measurements are available for comparison with the present results. The present study yields Cl--HPO42- mixing parameters for these three models that are needed for modeling the thermodynamic activities of solute components of natural waters and other complex aqueous electrolyte mixtures. (C) 2011 Elsevier Ltd. All rights reserved.