Isopiestic measurements have been made for aqueous solutions of the common sodium cation mixtures of NaH2PO4 and Na2HPO4 at T = 298.15 +/- 0.01 K, at NaH2PO4 ionic strength fractions y = (0, 0.24851, 0.49862, 0.74544, and 1), where the ionic strength fractions were calculated by assuming complete electrolytic dissociation of NaH2PO4 as 1:1 and Na2HPO4 as 2:1 electrolytes; CaCl2(aq) was used as the reference standard solution. Model parameters for an extended form of Pitzer's ion-interaction model and also for the Clegg-Pitzer-Brimblecombe equations based on the mole-fraction-composition scale were evaluated at T = 298.15 K for both NaH2PO4(aq) and Na2HPO4(aq) using the isopiestic results from this study (17 values each) together with numerous critically assessed osmotic coefficients for both electrolytes taken from the published literature. The thermodynamic models for NaH2PO4(aq) extend to m = 7.5 mol.kg(-1), whereas those for Na2HPO4(aq) extend to m = 2.6050 mol.kg(-1), which is well above the solubility limit for the thermodynamically stable phase Na2HPO4 center dot 12H(2)O(cr). The 51 osmotic coefficients for the ternary mixtures were treated with these two models together with Scatchard's neutral-electrolyte model; one previous set of osmotic coefficient values for {yNaH(2)PO(4) + (1 - y)Na2HPO4}(aq) mixtures was found in the literature [Scharge, T.; et al. J. Chem. Thermodyn. 2015, 80, 172-183], and hence an analysis and comparison were made of our results with theirs.