The molality-based osmotic coefficients of aqueous mixtures of K2SO4 and K2HPO4 have been measured at T = (298.15 +/- 0.01) K by the isopiestic method, at K2SO4 ionic strength fractions y = (0.21008, 0.39887, 0.60657, 0.80517, and 1) over the ionic strength range (1.3167 to 1.9587) mol . kg(-1), using KCl(aq) as the reference standard solution. 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 solution 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. Model parameters for K2SO4(aq) at T = 298.15 K were also evaluated using the present results along with seven sets of osmotic coefficients taken from the published literature. For the {yK(2)SO(4) + (1 - y) K2HPO4}(aq) system, few or no mixing terms were needed for the three models because the ionic strength dependences of the osmotic coefficients of K2SO4(aq) and K2HPO4(aq) are very similar and their curves nearly parallel, as are those of the mixtures which are intermediate and nearly equally spaced as a function of the ionic strength fraction. (C) 2014 Elsevier Ltd. All rights reserved.