Isopiestic vapor-pressure measurements were made for K2SO4(aq) from (0.228 to 0.740) mol(.)kg(-1) at 298.15 K and from (0.200 to 0.957) mol(.)kg(-1) at 323.15 K, using NaCI(aq) as the isopiestic reference standard. These isopiestic measurements extend to saturation at 323.15 K and to slightly above saturation at 298.15 K. There are no previous isopiestic studies for K2SO4(aq) at 323.15 K. Thermodynamic properties from the literature for the potassium sulfate + water system and the present measurements were used to generate equations that represent the thermodynamic properties of this system from the freezing point of the aqueous solution to 500 K. The measured values included for least-squares estimation of the model parameters spanned the temperature range approximately 271 K to 530 K for K2SO4(aq).