Isopiestic molalities of aqueous solutions of Na2HPO4, K2HPO4, NaH2PO4, and KH2PO4 have been measured at intervals from T = 383.15 K to T = 498.15 K, with NaCl (aq) as the isopiestic standard for the calculation of osmotic coefficients. Osmotic coefficients for these electrolytes, already low at T = 298.15 K, decreased even more at elevated temperatures, becoming quite low with equilibrium isopiestic molalities as large as 15 mol.kg(-1). Minor modifications of the standard ion-interaction model were necessary in order to obtain an acceptable fit of the experimental osmotic coefficients. Stoichiometric activity coefficients were computed from parameters resulting from the least-squares fit, which was quite good, of the osmotic coefficients. In the case of the di-hydrogenphosphates the relative order of the osmotic and activity coefficients reversed at the higher temperatures, which was not observed for the mono-hydrogenphosphates.