The molality-based osmotic coefficients of aqueous mixtures of KBr and K2HPO4 have been measured at T = (298.15 +/- 0.01) K by the isopiestic method at KBr ionic strength fractions y = (0.18328, 0.38241, 0.58031, 0.79186, and 1) and over the ionic strength range (2.5452 to 10.0418) mol . kg (-1), using KCl(aq) and CaCl2(aq) as the reference standard solutions. These experiments were performed using glass sample dishes because preliminary experiments showed that some corrosion of our usual gold-plated silver dishes occurred with dissolution of silver, presumably by "pinhole" corrosion at imperfections in the gold layer. These osmotic coefficients 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 this ternary system. The present assessment yields mixing parameters for these models. Model parameters for KBr(aq) at T = (298.15 +/- 0.01) K were also evaluated using the present results along with four sets of osmotic coefficients taken from the published literature. For the {yKBr + (1 -y)K2HPO4}(aq) system, the model with five Scatchard mixing terms gave a significantly better representation of the osmotic coefficients than any of the other models and is therefore recommended for calculation of osmotic and activity coefficients. (c) 2013 Elsevier Ltd. All rights reserved.