Equilibrium water vapor pressures over supersaturated aqueous ammonium sulfate have been determined from 278.15 to 313.15 K using an electrodynamic balance. Partial molar enthalpies of water (L(1), to similar to 25 mol kg(-1)) and partial molar heat capacities (J(1), to similar to 8 mol kg(-1)) have been calculated from the measurements, together with enthalpies of solution, heat capacities, and boiling point elevations. The results-have been used to parameterize a mole-fraction-based activity coefficient model. Equilibrium constants for the reaction (NH4)(2)SO4(cr) reversible arrow 2NH(4(aq)(+)) + SO4(aq2-) have been evaluated from 373.15 K to the eutectic point (5.00 mol kg(-1), 254.21 K). The model satisfactorily represents water vapor pressures and saturation with respect to solid phases ice and ammonium sulfate from 253 to > 373.15 K and up to 8-10 mol kg(-1), and water vapor pressures from 278.15 to 313.15 K and up to similar to 25 mol kg(-1).